Unconditionally Stable Floating Offshore Platform

This patent is interesting since one of the inventors (Arcandra Tahar M.Sc, Ph.D) happened to be an Indonesian minister of energy and mineral resources for less than 1 month (27 July 2016 – 15 August 2016). My curiosity drives me to search patent under his name.  Since this patent is related to oil and gas industry, I would like to share the idea behind the invention which in my opinion is simple enough to develop a strong offshore platform which will stand against a hurricane.

The patent is related to mega-structure (offshore platform) for oil and gas well drilling, production and operations.  Some offshore platforms have several decks. Those decks facilitate many operation activities and accommodation for crews. An offshore platform could have four or more wells and complete production facilities. It also has its own energy sources (gas or diesel gen set) for electricity. Figure 1 shows an offshore platform in operation and Figure 2 depicts a failed offshore platform when the loads are exceeded over the designed parameters.

 

Figure 1: Offshore platform in operation

 

The patent can be applied to floating offshore platforms such as semi-submersible platforms and tension leg platforms. Both kinds of platforms have decks for oil and gas operation activities and hull which keep the decks above the sea surface. The hull draft (the hull section below sea surface) will keep the platform afloat. It is designed to haul the payload required for the operations and to stand for any natural load such as wind blow and the water wave. The payload could be maintained by platform crews but the natural load sometimes is heavier than originally designed. This is the basic idea behind the invention which is to compensate the excessive natural load such as Catherine Hurricane by adding adjustable equipment on the hull. Conventional offshore platforms do not have the equipment which will capsize when the loads against the platform exceed the designed parameters.

Figure 2: Failed offshore platform

Figure 3 shows the column of conventional offshore platforms without compensation equipment. The platform will capsize once the platform tilt more than designed angle (heel angle).

Figure 3: Platform column tilted more than designed angle and then capsize due to lack of compensation equipment

Figure 4 shows the column of the present invention which could be applied to semi-submersible and tension leg offshore platform. Once the instrument in the compensation equipment detects unstable condition (several degrees of tilt angle), the compensation equipment will either supply gas into the gas chamber or release the gas. The gas will push out sea water from the water chamber or allow sea water entering the water chamber. By pushing out and adding in sea water into the chamber, the offshore platform will stabilise against loads.

Figure 4: Offshore platform with present invention column

The size of the gas and water chamber depends on several parameters such as payload, the spacing of the columns, the area of the columns, the draft, the freeboard, metacentric height and natural load. All parameters are evaluated during the designed process of the platform to eliminate the possibility of capsizing. The wind speed of 100 knots is usually used as a natural load.

Determining Distribution of Injection Fluid

It is important to determine fluid distribution after injecting a certain volume of fluid into a reservoir. Material balance method combined with Darcy’s and EOS can be used to calculate fluid distribution analytically or numerically. By knowing porosity and permeability distribution in the reservoir, one can predict prefer fluid front direction. The information is important for successful field flooding.

Some companies have done pilot flooding before full-field flooding project. This is to ensure the oil recovery can be achieved efficiently and effectively. The more robust technique has been done to acquire complete data of fluid distribution by means of 3D seismic.

Logging tools also have been used to log fluid distribution around wellbore such as C-O log. Frequently, the operators find oil potential behind water coning wells.

Figure 1: Surface measurement

In the present patent by SLB, an electromagnetic logging tool is used to determine fluid distribution around the wellbore. The logging tools are run first to get baseline data. Then electrolytic fluid containing electrically conductive particles (metallic powders or carbon nanotubes) is injected into the reservoir. The logging tools measure electromagnetic response as a function of time. The key point here is the electrically conductive particles must generate sufficient electrical resistivity (conductivity) contrast between the injected fluid and the existing fluid.

Figure 2: Fluid front direction

Measurements also can be run on the surface (Figure 1). Two transmitters and one receiver are required to produce vertical areal fluid distribution. The distance between the transmitters is commensurate with the vertical depth of investigation. More transmitters and receivers on the surface are required for other directions. By combining in-well logging data and surface measurement data, it is possible to map 3D fluid distribution as a function of time. By doing so, one can conclude preferable fluid front directions (Figure 2).

Oil and Service Company Research in 2013

The following are the figures of research activities in oil and gas industry:

1. Schlumberger receives the highest number of granted patent (Figure 1). Mostly in reservoir evaluation and engineering such as interpreting well test measurements and estimating fluid flow in a reservoir.
2. Chevron has the most active research activities among oil and gas producers (Figure 2).
3. Research activities of oil and gas company are mostly in petrochemicals such as continuous catalyst activator and olefin oligomer composition.
4. Baker Hughes and Halliburton researches are mainly in drilling and completion such as modular hybrid drill bit and polysaccharide based cement additives.
5. The highest increase in 2013 granted patent is 102 granted patents received by Halliburton (Figure 2).
6. Overall, mostly granted patent is in drilling and completion activities (29%, Figure 3).

Figure 1: Granted patents in 2013

Figure 2: Three years research activities

Figure 3: Research activities in 2013

Location of Bypassed Hydrocarbons

In heterogenic reservoirs such as carbonate reservoirs, bypassed hydrocarbon  is typically  left  somewhere in the reservoir.  We can easily identify bypassed hydrocarbon around  low recovery wells (FIGURE 1). The blue area in FIGURE 1 is the region of low recovery wells  whereas green area is the region of high recovery wells. A strong signal is indicated in well 15 when well 27 is injected however the signal is weak or none in well 20 and 30. To develop the blue area, more wells  are required to recover all bypassed hydrocarbon compared to green area. Drainage radius in the green area could reach 300 m compared to the blue area of 100 m. Blue area is more suitable developed by using horizontal wells or fracturing.

FIGURE 1: Low and High Recovery Regions

In the current patent by Chevron, bypassed hydrocarbon regions are identified by using permeability models and connectivity models. Those models are built for each production wells and injection wells. Both results from production and injection wells are combined to show long time of flight (TOF) regions which are identified as bypassed hydrocarbon regions. The modeling and overlay are done by using computer and fast matching algorithm. The permeability data can be determined by core samples and well logs. Seismic data is also available for areal permeability modeling. The results of the modeling and overlay are drilling proposal for injection and production wells which are included horizontal wells.

Oil and Service Company Research in 2012

Compared to previous year, granted patents in 2012 for 6 major oil and service companies increase 9%. Schlumberger Technology Corporation (Sugar Land, TX) was still the most active assignee (Figure 1). Halliburton booked for the highest number in additionally granted patent with 50 granted patents more than 2011.

Figure 1: Granted patents in 2012

Drilling and completion category were the most active research (Figure 2). Baker Hughes contributes the most with more than 200 patents in drilling and completion.

Figure 2: Research activities in 2012

Schlumberger spent most of their research budget for reservoir evaluation and engineerings such as measuring streaming potentials, determining formation parameter and directional resistivity measurements. Baker Hughes and Halliburton were both busy in drilling and completion researches such as Hybrid bit and Acid-soluble cement compositions.  Similar to the previous year, Chevron, Shell and BP focused in Petrochemical  such as High octane aviation fuel composition, aromatics hydrogenation catalyst, and Carbonylation process.

PRODUCE HYDROCARBON LIQUID ONLY

Flaring associated gas is a common practice in oil field operation. The main purpose of flaring gas is to convert  ignited gas to un-ignited gas (CO, CO2, H2O). By doing so, operator prevents uncontrolled fire.

Thousand cubic feet of gas are burnt every day. The burning produces CO and CO2 as major air pollutant into the atmosphere. Oil field operators opt burning gas because difficulties in utilizing the gas and no limitation yet to burn gas.

Nowadays, global warming and expensive fossil fuel issues are rising. People are pushed harder to reduce air pollution and utilize fossil fuel as effectively as possible. The idea is to convert flare gas to liquid (GTL) and produce hydrocarbon liquid only, no more flaring gas and wasting unutilized fossil fuel in oil field operation.

Converting waste gas to useful hydrocarbon has been started since long time ago. The technology was founded by Franz Fischer and Hans Tropsch in the 1920s and became popular as FT synthesis. FT synthesis converts syngas (CO, H2) to synthetic fuel.  It uses High Pressure High Temperature (HPHT) equipment and high power to convert CO back to hydrocarbon. Germany used the technology to fulfill their need in liquid hydrocarbon fuel during the world war. There was no significant advancement in the technology since that.

Due to scarcely of hydrocarbon fuel, people start using FT synthesis to convert waste gas to valuable hydrocarbon after more than 50 years since founded.  HPHT equipment and high power requirements in FT synthesis bring an idea of finding alternative technology to convert waste gas. Some scientists still use some parts of FT synthesis and make better processes in preparing syngas (steam-methane reforming, partial oxidation, autothermal reforming, gas heated reforming). Others do not use FT synthesis at all.

The following patents convert hydrocarbon gas to hydrocarbon liquid by using a high shear device and chemical reaction. Both technologies can be used by oil field operators who want to produce hydrocarbon liquid only. For economic valuation, they can contact CompactGTL, headquartered in the UK, which can construct GTL facilities as required such as mobile skid facilities.

System and process for production of liquid product from light gas

The process mechanically broke source gas to a smaller molecule or radical molecule. The molecules disperse in a liquid medium such as water in the form of bubbles. To achieve very small bubbles (0.1 to .5 micron m), a high shear device (HSD) and catalyst are required (Figure 1). The HSD can generate 20000/s of shear rate. The shear rate and catalyst (ruthenium) can be chosen selectively to make certain products. Multi stage HSD can be used for high production rate.

Figure 1: Converting hydrocarbon gas by using high shear device

The product from HSD is stable for 15 minutes and flowing into the reactor for recombination. The reactor will recombine radical molecules to produce liquid hydrocarbon. Produced hydrocarbon liquid will flow to a storage tank and the remaining gas is trapped in a condenser and flowed back to HSD. Some produced hydrocarbon liquid (fuel) can be used to run the equipment (HSD, reactor, pumps, and condenser).

Process For Converting Gaseous Alkanes to Olefins and Hydrocarbon Liquids

The process in this patent converts hydrocarbon gas to hydrocarbon liquid chemically. Bromine can break hydrocarbon gas at a lower temperature compared to other methods such as steam-methan reforming which requires temperature more than 600 C. This process can be used with less expensive equipment and more efficient energy.

Figure 2: Converting hydrocarbon gas by using dry bromine vapor

The gas feed will be mixed with dry bromine vapor in a reactor at a temperature between 250 and 600 C (Figure 2). The reaction will produce alkyl bromide and hydrobromic acid vapor. The products will flow to the second reactor for catalytic reaction (zeolite as the catalyst) to produce hydrocarbon liquid.  A scrubber is used to clean-up hydrocarbon liquid.  Excess of hydrobromic vapor will flow to reconditioning facilities for treatment to recover dry bromine vapor.

Oil and Service Company Research in 2011

In 2011, 6 major oil and service companies produce more than 1000 granted patents.  The company with the highest number of invention is Schlumberger Technology Corporation with more than 400 granted patents (Figure 1). Major oil companies produce less granted patents. Furthermore, oil company such as Chevron U.S.A. Inc. (San Ramon, CA) only receive less than 200 patent certificates.

Figure 1: Granted patents in 2011

The inventions are mostly in production and operation activities (Figure 2). Schlumberger contributes the most with 173 granted patents span from production management, measuring flowrate, cable, fluid filter, well treatments to ICD. Major oil companies only have 66 patents in the activity.

Figure 2: Research activities in 2011

Schlumberger as a logging company has been done research mostly on reservoir evaluation and engineering (188 granted patents). Forty-five patents are in the Logging, formation  and reservoir evaluation such as logging tool, induction resistivity tool, formation evaluation system, and integrated reservoir optimization. Besides, people in SLB research center are also loaded with many novel ideas in drilling and completion activities such as steerable drilling system and completing a multiple-zone well.

Baker hughes and Halliburton look similar to Schlumberger. Their research activities are mostly in drilling, completion, production, and operation.

Contrary to service companies, major oil companies are busy on petrochemical research activities. More than 35% of the research that turn out to be granted patent are on petrochemical inventions such as lubricating oil compositions and low sulphur alkylate gasoline fuel. Shell got 60 granted patents on petrochemical inventions out of total 162 patents.

Near Wellbore Stimulation

I believe there are many methods and processes to stimulate reservoir near the wellbore. Among many popular methods (acidizing and fracturing), microbial stimulation and heat-vibration are still unpopular. Usually, the operator tries to avoid these methods since it is almost impossible to control the processes behind casing.

1. MICROBIAL STIMULATION

Bacteria can live in a harsh condition such as in oil reservoir. Bacteria have a very simple cellular structure (FIGURE 1)that helps it to stay dormant in reservoir pore. By improving the environment condition, bacteria can “wake-up” from a dormant state to living micro-organism.  In very satisfaction environment, bacteria can grow by cell division and double in number as quickly as 9.8 minutes. Dormant anaerobic bacteria in the reservoir will stay dormant in the produced water. By sampling the water and analyzing in the lab, the bacteria can be identified and then the resource substances can be formulated to grow the bacteria. If there are no suitable bacteria in the reservoir to disclog oil, bacteria that will live mutually with residence bacteria will be injected from the surface within injected water along with nutrients.

FIGURE 1: Bacteria with simple cellular structure

The microbe has certain criteria to grow. Generally, the microbe can live below 80 ᵒC and 2400 psi. Salinity and pH also affect the optimum condition to grow microbe.

The substances that will unclog the oil and make it more movable are bacteria secretion. The secretion acts similar to  a chemical surfactant that influences surface tension oil-water and oil-rock. In very favorable condition, bacteria will multiply faster and produce more secretion. Phosphate must be supplied as part of bacteria nutrition besides of oil. As one cell organism, optimum thermodynamic activity in the form of oxidizing – reducing reaction will promote faster multiplying. Anaerobic bacteria such as sulfate reducing bacteria (SRB), nitrate-reducing bacteria (NRB), iron-reducing bacteria (IRB) and acetogenic bacteria will act as electron donor while Nitrate that is supplied along with injected water will act as an electron acceptor. The inventor believes vitamins such as  B12, biotin, folic acid, nicotinic acid, aminobenzoic acid, calcium pantothenate, pyridoxine HCl, riboflavin , thiamine, thioctic acid will promote better condition for bacteria to multiply.

Anaerobic bacteria that eat oil are more favorable than that does not eat oil. By eating oil, nutrient supplied along with injected water will be less. The rate and concentration of nutrients and vitamins must be determined in the lab. The inventor suggests the rate between 0.1 and 15 m/day and concentration between 1 and 1000 m g/l. The inventor also specifically avoids the use of oxygen.

2. HEAT-VIBRATION STIMULATION

Reacting two chemical substances in the reservoir is the idea. The reaction will produce micro-explosion (vibration) and heat. The chemical substances must endure at reservoir P/T before reaction. A spacer such as water, brine or carbon tetrachloride, is injected between chemical substances to avoid premature reactions.

The main chemical substance in the invention is pentaethylene hexamine -3CO (PEH-3CO, FIGURE 2). PEH-3CO (polymer) reacted with acid will produce micro-explosion, heat, and CO2. The following is the stoichiometry of PEH-3CO / acid system:

PEH-3CO + nHX (acid) —-> PEH Hn + Xn- + 3CO2 + heat

FIGURE 2: PEH-3CO

Gas CO2 will increase pressure. Heat will decrease oil viscosity and vibration of micro-explosion will release trapped oil. Overall, the stimulation will increase oil recovery.

The vibration intensity and duration of micro-explosion can be controlled by the type and structure of the chemical substances used. The type and structure of the chemical substances must be evaluated in the lab. Compatibility check must be run to avoid precipitation in the reservoir due to negative ion Xn- since formation water contains several salts.

WATER FLOODING

Water flooding for EOR candidate reservoirs is important to be done. Some literature suggests that solution gas drive reservoir is the best candidate for water flooding. The water will sweep remaining oil which is left by primary recovery between wells due to lack of reservoir pressure. However, there is a possibility that the reservoir pressure is still high but the oil is immobile due to low mobility and loss solution gas during depletion period. By doing water flooding, pay continuity will be concluded which will reduce risks during EOR operation.

The target reservoir contains 31.93 MMSTB of oil with 10% oil recovery after 27 years of production.  The gross pay of the reservoir which is mostly shaly limestone is 45 m average.  The primary drive mechanism in the reservoir is fluid expansion drive (solution gas drive) with very low water influx. Production from the reservoir is characterized by high initial rate with high rate decline. Currently, mostly wells producing from the reservoir suffer water blocking where water is encroaching the wells and is saturating around the wellbore.

The oil has 35 API gravity with 211 scf/stb cumulative GOR (black oil). Initially, the oil moved 2.1 times slower than water. At pressure around 900 psi, the oil moved slower and  was left by water which saturated the vicinity of the wellbores.  The wellbores have then suffered high water cut production (more than 95%) until a economic limit is reached. However, when a new well is drilled in new location (such as only 15 meters away, well C1 on Figure 5), the  well produces fluid with water cut much lower (60%) even the perforation is lower than the 95% water cut wells (well B and B1 on Figure 5). This indicates that much oil which cannot move to water-blocked wells is still left between wells.

Water flooding model

The model is based on frontal displacement theory which was developed by Buckley and Leverett in 1942. The fractional flow equation and frontal advance equation in the theory are applied for radial flow model. Both equations are presented here:

Fractional flow equation:

Frontal advance equation:

To solve fractional flow equation, data permeability vs water saturation is required from core analysis (Figure 1). For simplifying, capillary pressure effect is neglected.  Both graphs of fractional flow equation and frontal advance equation are plotted on the same scale (Figure 2). The effect of injection is evaluated in three phases:

1. Phase 1: Solving water blocking around wellbore of producers, no noticeable water cut change in the producers.

2. Phase 2: Stabilizing water cut, water cut in the producers gradually decrease.

3. Phase 3: Stabilized water cut, water cut in the producers relatively constant until water breakthrough.

Phase 1 will be last for 7.6 days without noticeable water cut change in the producers. Water cut will gradually decrease to 73%. This production profile will be stable until water breakthrough or 314 days. Figure 3 shows cross-section front movement and Figure 4 shows radial front movement.

Figure 1: Permeability vs water saturation

Figure 2: Solving fractional flow equation and frontal advance equation

Figure 3: Cross section front movement

Figure 4: Radial front movement

Effect of well A (injector)

Refer to top structure map on Figure 5. The structure is relatively plate with around 250 m of distance between wells. Well A have been injected for quite long. Total 261 Mbbls of water have been injected since 2009 with an average rate of 575 bwpd. The injection phases in well A1 and A3 is not distinguishable. However, based on the model above, phase 2 had been finished in 30.5 days or after 101 Mbbl (5.1% PV) of water injected and phase 3 have remained until water breakthrough reached or after injecting 1039 Mbbl (53.1 % PV) of water. Rate declines on Figure 6 and Figure 7 show clearly the effect of injection in well A where both rate declines are negative. Water cut of both wells tend to decrease to 73% and oil productions keep increasing. Noncontinuous injection to well A due to unreliable production facility results in low horizontal sweeping efficiency. “Stop and go” situation during injection with low injection rate (575 bwpd average) has allowed water to escape to aquifer naturally and leaves oil on top building water-oil layer in the reservoir.  Continuous injection with a high rate (2000 bwpd) will solve the problem.

Figure 5: Top structure

Figure 6: Rate decline well A1

Figure 7: Rate decline well A3

Estimated production

Current field production is 270 bopd with only well A as an  injector. By converting well B, C and D into an injection well, the field will lose 30 bopd. After 30.5 days, the production will climb up to 560 bopd where every injection cluster contribute between 50 and 125 bopd each.

Future development

After injecting water for 314 days or 53.1% PV, all producers will water out. Continuing water injection will uneconomically visible. However, from the previous project, pay continuity had been concluded. Moreover, well A as injector connect to well A1 and well A3 as previously discussed. IOR or EOR is the next step for exploiting the reservoir. By adjusting the completion of wells to make the injector closer to producers by means of horizontal radial drilling will improve oil recovery (IOR). Adding chemical such as surfactant into injected water will enhance oil recovery (EOR). Injecting water+chemical immediately after 15% PV of water injected will decrease pumping cost and processing costs.

In the current patent by BP Exploration Operating Company Ltd.,  water with certain ionic concentration will give better performance in case of the horizontal and vertical sweep. It is concluded that 500 to 5000 ppm of certain ionic concentration will increase oil recovery. To achieve such concentration with certain ionic concentration, a reverse osmosis plant,  and high salinity source water are required. The plant will need at least 0.1 MPa of pressure higher than the  osmotic pressure of the membrane to get reverse osmosis effects. To fulfill 2000 bwpd injection rate, around 30 square meters of an osmotic membrane are needed.

OPENING ON CASING FOR RESERVOIR MONITORING

Various logging tools deploy into the borehole to gather various reservoir properties before steel casing run. Some logging tools which use electric signals to measure reservoir properties must be run in open hole. The logging tools can not be used in cased hole due to the restriction of steel casing. Reservoir properties such as water saturation and porosity must be concluded from the logging data as an initial condition of the reservoir.

The casing is run subsequently to secure the hole wall from collapsing, steel casings are used mostly. Then cement is placed between casing and borehole to isolate productive zones hydraulically. The casing and cement are perforated at the productive zone to allow reservoir fluid flowing into the well.

In a horizontal well, a slotted liner is preferable than casing tubular since the leg (horizontal section) is only connected to one reservoir . The slotted liner is required to prevent the borehole from collapsing and the slots are designed to prevent sands flowing into well.

During the well life, fluid saturation around the well or between wells is changing. Re-running electrical logging tools to measure fluid saturation are impossible. Pulsed neutron logging  which measures carbon-oxygen ratios is run to assess the changing. But this method has a shallow depth of investigation and low accuracy in low porosity reservoir.

The question comes up whether there is any possibility to pass electric signal (EM signal) through slots and measure reservoir properties as done in open hole logging. The answer is yes. Now, the challenge is to make the slot through the casing and run suitable electric logging tools.

The inventors protect their idea in the form of a method (procedures) and system (tools).  Making the opening (slots) is a very important part of the method and the system. The slots can be pre-installed on the casing before running or made using mill-cutter, perforating gun, sandblast cutter or other means. At least one transmitter and one receiver are required, more is better.

Figure 1: Pre-installed slots on casing

Figure 1 shows pre-installed slots on the casing. The insulator which is made from a material that transmits EM radiation covers the slots to provide isolation during cementing.

Figure 2: Logging in water coning well

Re-logging producing wells can give information about fluid distribution behind casing or between wells. Re-logging periodically to monitor the reservoir will give information any changing during the life of the well. Sometimes, in high water cut wells, oil is still unproduced due to water saturated zone. Figure 2 shows the logging result. Instruments B and C gives difference results for deep and shallow surveys which conclude oil left due to water blocking around well. Immediate remediation can be taken accurately to allow oil flowing into the well.