Liaohe heavy-oil field pilot improves gravity-assisted steam flooding
Yongjian Liu
Lingling Guo
Shaobin Hu
Northeast Petroleum University
Daqing, China
Yuchuan Cai
PetroChina Liaohe Oilfield Co.
Panjin, China
A new gravity-assisted steam flooding technique helped control steam-channeling problems in the Liaohe heavy-oil area in China. Reservoir fluid-mechanics analysis showed that a large dip angle hinders steam flooding's effectiveness.
The two-pronged technique uses gravity to enhance flooding, reducing the steam quality in high-position steam injection wells and increasing hot-water displacement efficiency. It also reduces steam-injection intensity in low-position steam injection wells to weaken buoyancy.
Field testing in Liaohe's Qi 40 block yielded good results, indicating the process can be extended to other blocks having an oil-layer dip angle similar to the 65 test wells.
A computer simulation and field testing showed optimized parameters with steam quality of 30% in the upper-steam injection wells and a 50% reduction of steam-injection intensity in lower-steam injection wells.
Steam quality is the mass percentage of dry steam in the wet steam. Steam generally includes a certain amount of hot water drops: steam quality is 90% if 10% is water.
Block Qi 40 yields 14.5° gravity oil with 2,639-cp viscosity at ground conditions. Porosity averages 32.5% with horizontal permeability averaging 2.03 darcy. Initial oil saturation is 51%.
Large-dip angle
Steam-flooding problems consistently develop in middle- and late-production stages in reservoirs having dip angles greater than 10°.1-3
Injected steam tends to flow upward, arriving prematurely at updip production wells and causing steam channeling. Poor response from the downdip portion of the reservoir leads to production declines.
Condensed hot water flows toward the downdip. Heated oil flows toward the lower-position production wells. Production is relatively high for wells in the downdip part of the reservoir. The overall heat utilization rate of the flooding process is low.4-7
A steam-injection well drives hot-water flooding to production wells. Operators lack an ideal method to use dip angle and gravity to improve steam-flooding efficiency in large-dip reservoirs.7 8
The Qi 40 block includes 149 groups of steam-flooding test wells. The oil-layer dip angle is more than 15° in 22 well groups. About one-third of the 149 groups exhibited a dip angle greater than 10°.8 9
Large-pressure gradients between injection wells and production wells affect steam migration. The pressure gradient lessens farther from the production well or injection well (Fig. 1). Increased distance enhances the effect of buoyancy in the reservoir, which influences steam migration along the oil-layer direction as do capillary force and pressure gradient (Fig. 2).
Interfacial resistance in the updip direction of the reservoir hindered steam-flow direction. Buoyancy and pressure gradient were driving forces.
Equation 1 shows buoyancy's effect on unit volume of steam along the reservoir direction (Fig. 3). Equations 2-3 show steam's effect along the oil-layer direction.
Buoyancy drove oil and steam along the updip direction of the oil layer, enhancing production. One buoyancy disadvantage was its aggravating of steam override at certain injection intensity levels, leading to channeling or premature steam breakthrough.
Pressure gradient drives steam migration along the oil layer's downdip direction. Interfacial resistance and buoyancy work against the steam flow. Fig. 4 shows the force analysis of steam.
Equations 4-5 show the forces of interfacial resistance, buoyancy, and pressure gradient along the oil-layer direction.
A large pressure gradient means that the buoyancy effect and interfacial resistance are no longer apparent along the downdip direction of the oil layer in the vicinity of the steam injection well or production well. The pressure-gradient effect gradually declines with increased distance between the wells.
A minimum pressure-gradient value exists in the transition zone from the steam injection well to the production well. Steam has difficulty moving downward when the minimum pressure gradient cannot counterbalance the resistance of the capillary force and buoyancy. Downdip production wells respond poorly.
Equations 6-8 show the critical oil layer dip angle. Qi 40 Block has a critical oil layer dip angle in the range of 20-30°.10 11
The downdip oil layer responds poorly to a steam-injection well between two production wells if the oil layer dip angle is greater than the critical angle. Steam flooding effectiveness is poor for production wells in the oil-layer's lower portion.
Theoretical analysis determined the oil layer's dip angle has a great negative influence on steam flooding.
Technical principle
Fig. 4 shows that if the steam injection well is the dividing line along the updip oil layer direction then the interfacial resistance is opposite to the steam migration direction, restricting steam flow.
Pressure gradient is the driving forces of steam migration along the downdip oil layer direction. Interfacial resistance and buoyancy resist steam migration. The oil layer needs a uniform downward pressure gradient to improve steam flooding.
Reducing steam quality in the upper row of steam-injection wells requires increasing the pressure gradient along the downdip direction using hot-water gravity.
Steam-injection intensity drops in the lower row of steam-injection wells, weakening the steam's buoyancy effect along the updip. A resulting steam-quality reduction in the upper steam-injection wells increases the injected hot fluid's gravity pressure.
This pressure increase can reduce the gravity-pressure difference from the steam-injection pressure gradient in the lower injection wells or change the direction of the resistance, yielding a more downward pressure gradient. The ultimate goal is to increase the steam injection wells' downdip pressure gradient, enhancing oil displacement by hot water.
Oil companies can reduce steam quality in high-position steam injection wells to control the updip steam sweep and increase the downward hot water gravity pressure in the high-position wells.
Operators also can reduce steam-injection intensity in the lower-position steam injection wells to reduce upward buoyancy and pressure gradient in the low formation. The gravity-pressure differential drives steam down to sweep the downdip oil layer.
Operational parameters determined through field testing and numerical simulation showed:
• This new approach is suitable for reservoirs having dip angles greater than 10° that experience steam-override problems from structural issues threatening to leave oil in the downdip from the steam-injection well.
• The technique has little effect on steam-flooding response time when bottomhole steam quality is less than 30%. Upper injection wells need a steam quality of 30%.
• A 50% reduction of injection intensity in the lowest row of injection wells can optimize reservoir engineering results using numerical simulation. Injection intensity refers to the injection rate per unit reservoir volume within the well group.
Technical parameters for Block Qi 40 oil layers having a 15° dip angle should be adjusted as follows:
• Keep the steam injection rate unchanged for the upper injection wells, decrease the boiler outlet steam quality to 45% from 75% to keep bottomhole steam quality at 30-35%.
• Decrease the injection rate by 20% for the middle steam-injection wells, reducing the injection intensity to 1.5 from 1.8 and increasing the production-injection ratio to 1.3 from 1.1. This maintains the steam quality.
• Decrease the injection rate by 50% for the bottom steam-injection wells, reducing the steam injection intensity to 0.9 from 1.8 and increasing the production-injection ratio to 1.5 from 1.1 to maintain steam quality.
Field testing
Field testing involved the large-dip angle (≥15°) area. These well groups are in the northern part of Block Qi 40. Steam-flooding involves the Lian II oil layer, which has 11 steam flooding well groups, a 0.36-sq km oil-bearing area, and original oil in place of 152 million tons.
The oil-bearing area consists of a monoclinal structure held by faults. The formation inclines from northwest to southeast with a large dip angle of 15-20°. The oil layer is 20-40 m thick. Small oil layers Lian II-1-1, Lian II-1-3, and Lian II-2-4 are very thick and are the main zones of steam flooding.
The operator switched to steam flooding during December 2006-March 2007. Before steam flooding was implemented, 29 of 45 production wells were operating and producing a combined 61 tons/day, with water production of 209 tons/day. The oil recovery rate was 1.2%, the ratio of production in a year relative to estimated reserves. Reserves recovery was 32%, the ratio of cumulative oil production relative to estimated reserves.
Production increased until December 2010 when it declined sharply from 200 tons/day to 150 tons/day.
Comparing the reservoir parameters of this large-dip angle area with the pilot and expanded experimentation areas in the early stage of breakthrough shows that the small oil layer of Lian II has entered the early stage of steam breakthrough.
Table 1 shows 11 well groups with 45 wells total. Steam channeling affects 22 wells, 16 wells are stable, 4 have production rising, and three others exist. The total production of the 45 wells is 165 tons/day.
The production of the 22 steam-channeling wells is 55.1 tons/day, 33.4% of total production. Production of the stable wells is 76.7 tons/day. These stable production wells will experience steam channeling and production decline unless action is taken.
The oil saturation of one well group in the experiment block was numerically simulated, which showed a steam override problem. The oil at the top was heated and displaced to the production well, but remaining oil mainly gathered in the downdip reservoir near the steam injection well.
Oil at the top of the oil layer is heat oil saturated, demonstrating steam-override problems. The top of the oil layer was heated to displace oil to the production wells. Remaining oil gathered in the reservoir's downdip below the steam injection well.
Reservoir-monitoring data showed the updip reservoir ,was mainly swept by steam. The thickness of the steam zone was about 60% of the oil-layer thickness.
Hot water sweeps about 78% of the oil-layer thickness, consistent with numerical simulation results.
Table 2 shows remaining oil in the small layers of Lian II. The area has an estimated 86.2 million tons in remaining reserves. The small layers of Lian II-1-1, Lian II-1-3, and Lian II-2-4 are the main steam-flooding candidates.
Reservoir numerical simulation and reservoir engineering provided the experimental parameters' basis.
The lower the steam quality, the higher the ratio of hot water in steam, and the faster the response of gravity oil displacement.
Fig. 5 shows the simulated relationship between steam quality and gravity-displacement response time. Steam quality less than 30% had little effect on response time and was used in the upper injection wells.
Researchers decreased the steam-injection rate of the lower row of injection wells by 50% while keeping the middle row's rate steady. Table 3 shows simulation results of various steam qualities including 30%, the best for use in the upper injection wells.
Table 4 shows the simulation results of the different injection intensities in the middle and lower rows of injection wells at 30% steam quality. Scenario 1 is the best. The gravity-assisted steam flooding process plays a positive role in enhancing oil recovery for about 2 years. Reserve recovery of 7.4% is possible. Fig. 6 shows the testing area's daily oil production.
Development Scenario 1 in Table 4 was implemented in 11 well groups. Steam injection into the 11 well groups decreased to 740 tons/day from 1,045 tons/day.
Analysis found that the pressure field changed in the middle two rows of wells. The response of the updip part within the well group decreased while the response of the downdip part increased. Total oil production of the well group increased.
Overall temperature changed using the new steam-assisted gravity drainage technique. The temperature dropped in the updip part and increased in the downdip part, which matched theoretical results. Oil production increased from 150 tons/day from before the experiment to a peak of 206 tons/day during the experiment.
Oil production increased 56 tons/day of which 39 tons/day was obtained from the middle two rows of production wells.
The new technique saved steam injection while increasing oil production. The oil-steam ratio increased from 0.13 to 0.18, saving $1.3 million.
Qi 40-H, a well in the middle of the test area, showed a weakening steam override along the updip direction. The wellhead temperature dropped.
Acknowledgments
The authors acknowledge financial support received from the National Science and Technology Major Project (2016ZX05012-001), the Technical Demonstration Project of Heavy, Ultra Heavy Oil Development in Xinjiang and Liaohe Oilfields (2016ZX05055) and the Open Project of EOR Key Laboratory of Ministry of Education (NEPU-EOR-2014-012).
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The authors
Yongjian Liu ([email protected]) is a professor at Northeast Petroleum University and a supervisor at the Educational Ministry for Improving Oil and Gas Recovery laboratory, Daqing, China. He holds a PhD (1993) in chemical engineering from Zhejiang University. He is an SPE member.
Lingling Guo ([email protected]) is an associate professor at Northeast Petroleum University College of Computer and Information Technology, Daqing, China. She holds an MS (2003) in applied computer technology from Daqing Petroleum Institute and is working toward a PhD in petroleum engineering.
Shaobin Hu ([email protected]) is an associate professor at Northeast Petroleum University College of Petroleum Engineering. He works in the Educational Ministry for Improving Oil and Gas Recovery laboratory. He received a PhD (2011) in oil and gas field development engineering from Northeast Petroleum University.
Yuchuan Cai ([email protected]) is an assistant director and senior engineer of the Huanxiling oil production plant at Liaohe field. He holds a BS (1988) in geology from the Changchun Institute of Geology. He designs oil and gas field development and researches enhanced oil recovery.