Extended Production Tests in the Liuhua 11-1 Reservoir
- C.P. Peng (Amoco Production Co.) | C.F. Bateman (Amoco Production Co.) | J.M. Kaffenes (Amoco Production Co.) | J.L. Yanosik (Amoco Production Co.) | Huan-Min Liu (Chinese Offshore Nanhai East Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- August 1994
- Document Type
- Journal Paper
- 169 - 174
- 1994. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 2.7.1 Completion Fluids, 1.6 Drilling Operations, 5.5.8 History Matching, 4.1.2 Separation and Treating, 5.1.1 Exploration, Development, Structural Geology, 4.2 Pipelines, Flowlines and Risers, 2.2.2 Perforating, 5.6.2 Core Analysis, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 5.6.4 Drillstem/Well Testing, 5.5 Reservoir Simulation, 1.14 Casing and Cementing, 4.6 Natural Gas, 1.10 Drilling Equipment, 5.5.2 Core Analysis, 1.2.3 Rock properties, 4.3.4 Scale, 2 Well Completion, 5.3.4 Integration of geomechanics in models, 3.1.2 Electric Submersible Pumps, 1.12.1 Measurement While Drilling, 5.1 Reservoir Characterisation
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Amoco Production Co. and Chinse Offshore Nanhai East Corp. have conducted three extended production tests (EPT's) on the Liuhua 11-1 reservoir offshore China in the South China Sea. The wells selected for these EPT's included a vertical well, a high-angle deviated well, and a long horizontal well. This paper presents results and analyses of these EPT's.
Amoco made its first oil discovery in the South China Sea during Feb. 1987. The discovery Liuhua 11-1-1A well, drilled in Contract Area 29/04 (Fig. 1), encountered a significant amount of hydrocarbon in a thick carbonate interval (Liuhua 11-1 reservoir). After the discovery, Amoco undertook an active appraisal program to determine the reservoir extent and characteristics and to provide data for predicting reservoir production performance. The appraisal program consisted of drilling four wells: Liuhua 11-1-3, 11-1-4, 11-1-5, and 11-1-6. Liuhua 11-1-3 and 11-1-4 are vertical wells in the eastern and western parts of the reservoir, respectively. Liuhua 11-1-5 is a high-angle deviated well drilled between Liuhua 11-1-1A and 11-1-3. Liuhua 11-1-6 is a 1,900-ft horizontal well drilled between Liuhua 11-1-1A and 11-1-4 (Fig. 2).
Geologic studies, well logs, and core data indicate that the Liuhua 11-1 reservoir has a significant volume of oil in place. The oil gravities range from 16 to 23°API, oil viscosities are between 40 and 130 cp, reservoir porosity is between 20% and 25%, water saturation is between 30% and 50%, and reservoir thickness is between 150 and 230 ft. The reservoir is highly stratified and is underlain by a large, permeable aquifer. Water influx from the aquifer is expected to provide the energy for oil recovery and to dictate the recovery performance of the reservoir.
Initially, we thought development of the Liuhua 11-1 reservoir was possible with conventional vertical well technology with multiple platforms. Because of the cost and risk involved in the development, however, we initiated three EPT's to evaluate the reservoir and to define the risks further. A vertical well, Liuhua 11-1-3, was the first well selected for an EPT. Analysis of EPT results indicated that development of the Liuhua 11-1 reservoir with conventional vertical well technology would not be economical owing to water coning and poor displacement characteristics. Preliminary calculations indicated that a smaller-scale system with horizontal or high-angle deviated wells would be needed for reservoir development. Horizontal and high-angle deviated wells have the advantages of increased productivity and reduced water coning because of the longer well length and reduced drawdown. To verify the deviated and horizontal well performance and to define the risk further, a deviated well EPT with Liuhua 11-1-5 was started in 1988 and a horizontal well EPT with Liuhua 11-1-6 was started in 1989. With these test results, the reservoir description for future performance predictions was quantified and a more appropriate development system was identified. This paper presents the test results and analysis.
The Liuhua 11-1 reservoir is ˜120 miles southeast of Hong Kong in the South China Sea (Fig. 1). The reservoir contains a biodegraded, viscous crude and is underlain by a large aquifer. Cores and well log data indicated that tight layers exist in the reservoir and between the reservoir and the aquifer. The primary objectives of the EPT's were to test the effectiveness of these tight layers in limiting aquifer influx and to evaluate the potential advantages of high-angle deviated and long horizontal wells in minimizing the adverse effects of water coning. From an operational standpoint, the purposes of the tests were to evaluate the viability of using a submersible pump in this reservoir and to determine whether separation of oil/water emulsions formed during production would be a problem. Results from these EPT's found no serious operational problems.
The contract area is on the plunge of the Dongsha Massif, a regional arch. Overlying the basement of the massif are clastics and carbonates of Miocene Age to recent time. Sedimentation began on the Dongsha Massif with deposition of the Upper Oligocene to lower Miocene Zhuhai formation, a transitional to paralic sandstone and shale unit. Overlying the Zhuhai formation are marine carbonates of the lower Miocene Zhujiang formation that have developed on the massif into a series of carbonate banks with local reefing. The Liuhua 11-1 discovery produces from local closures on a major Zhujiang carbonate bank that has been extensively leached to form an excellent reservoir. The Liuhua 11-1 reservoir was subdivided into six diagenetic units, Units A through F, on the basis of porosity, permeability, water saturations, and segregation of oils that have undergone varying levels of biodegradation. Two highly porous intervals (Units B and D) are separated by a low-permeability interval (Unit C). Unit D is separated from the aquifer below by another tight interval (Unit E). Unit C was formed by the exposure of the platform during deposition, and Unit E was formed by mechanical compaction and focused cementing during and after oil fill. Fig. 3 shows a cross section of these intervals.
The oil found in the Liuhua 11-1 structure has been biodegraded by aerobic bacteria supplied with oxygen from the surface by invading waters. As a consequence, oil gravities range from 16 to 23°API across the field. A petrophysics study indicated that crude properties are also influenced by the existence of an exposure surface in the middle of the reservoir (Unit C). The productive reservoir is underlain by a large regional aquifer that outcrops at the seafloor approximately 40 miles away. During history-matching of EPT performance, the aquifer was assumed to be infinite acting.
Drillstem tests (DST's) have been conducted on four wells in the Liuhua 11-1 structure. Well test permeabilities are from 1 to 8 darcies, which is consistently 30 to 10 times higher than core air permeabilities and not unusual in a vuggy carbonate. Isolated interval DST's on the Liuhua 11-1-4 and the Liuhua 11-1-5 indicate that the more aggressively leached "D" zone is two three times more permeable than the "B" zone. This contrast in permeabilities in these intervals is also seen in core permeability measurements.
Although the well tests showed high reservoir productivity, the recovery mechanisms and potential of the reservoir were uncertain because of the tight streaks above the aquifer. The volumetric and solution-gas-drive potential of the reservoir is low because of the low solution-gas/oil ratio of the reservoir crude. If the tight streaks are too tight to allow vertical communication, the reservoir may not have enough natural energy for oil production. If the tight streaks are not effective in limiting water influx, then water coning will have a significant effect on the recovery potential of the reservoir.
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