SACROC Unit CO2 Flood: Multidisciplinary Team Improves Reservoir Management and Decreases Operating Costs
- J.T. Hawkins (Pennzoil E&P Co.) | A.J. Benvegnu (Pennzoil E&P Co.) | T.P. Wingate (Pennzoil E&P Co.) | J.D. McKamie (Pennzoil E&P Co.) | C.D. Pickard (Pennzoil E&P Co.) | J.T. Altum (Pennzoil E&P Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- August 1996
- Document Type
- Journal Paper
- 141 - 148
- 1996. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.8.7 Carbonate Reservoir, 5.6.4 Drillstem/Well Testing, 4.1.5 Processing Equipment, 5.4.9 Miscible Methods, 5.4.1 Waterflooding, 4.1.2 Separation and Treating, 6.5.2 Water use, produced water discharge and disposal, 2.2.2 Perforating, 4.3.4 Scale, 5.4 Enhanced Recovery, 3 Production and Well Operations, 5.1 Reservoir Characterisation, 5.4.2 Gas Injection Methods, 5.7.2 Recovery Factors, 1.6 Drilling Operations, 5.1.5 Geologic Modeling, 5.3.2 Multiphase Flow, 1.6.9 Coring, Fishing, 1.14 Casing and Cementing
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In 1992, the economic viability of the SACROC Unit was somewhat uncertain. At that time, a multidisciplinary team was formed to improve operational efficiencies and reservoir performance. Better understanding of reservoir geology from detailed biostratigraphic analysis provided the framework to make effective changes. This paper discusses operational efficiency and reservoir exploitation projects implemented by the team.
The SACROC Unit of the Kelly-Snyder field is located in the Midland basin. The Midland basin is the easternmost of the Permian Basins of west Texas. This field is the largest of the many prolific, Late Pennsylvanian age carbonate buildups that comprise the Horseshoe Atoll. The field, discovered in 1948, encompasses 50,000 acres and contained an estimated original oil in place of 2.8 billion bbl. Waterflooding operations began in 1954 and CO2 flooding began in 1972. Cumulative recovery has been more than 1.2 billion bbl. The field contains approximately 1,600 wells with about 400 active producers and 240 active injectors. Recent changes implemented by the team have significantly improved operational efficiency at the unit. In addition, recent geologic investigations have finally begun to unravel the complex stratigraphy of the reservoir.
Previous publications have documented the discovery of the Kelly-Snyder field, formation of the unit, completion of geologic and reservoir studies, and implementation of water- and CO2-flood projects at SACROC.1-8 Pennzoil acquired an interest in the field and became SACROC Unit operator beginning in late 1992. Shortly thereafter, a multidisciplinary team of reservoir engineers, geologists, production engineers, a facility engineer, and field operation personnel was formed and challenged with revitalizing the unit.
The SACROC Unit is situated on a prominent geologic feature named the Horseshoe Atoll (Fig. 1). The Horseshoe Atoll is a Middle Pennsylvanian through Early Permian age, isolated carbonate platform. During the Early Pennsylvanian, the Horseshoe Atoll was a broad platform, nearly circular in shape. However, beginning in the Late Pennsylvanian, the Midland basin began to subside rapidly. Subsequent tilting of the platform and drowning of the interior of the Horseshoe Atoll resulted in its characteristic arcuate or horseshoe shape.7
The SACROC Unit reservoir is a north-south trending carbonate buildup with a slight dogleg to the west (Fig. 2). The northern half of the unit is structurally highest, dips steeply to the west and east, and contains the thickest portion of the reservoir. To the south, the reservoir dips steeply, then flattens out to a broad, relatively flat platform. Along the eastern flank of the platform there is a trend of areally restricted "patch reefs." Overlying the entire structure is a thick sequence of dark black, organic-rich basinal shale. This thick sequence of shale forms both the seal and the source for the hydrocarbons trapped in this reservoir.
On discovery in 1948, the reservoir was originally described as a thick carbonate reef. At the time, distribution of depositional facies and attendant porosity trends in complex carbonate reservoirs were poorly understood. Consequently, the original development of the field did not account for the complex stratigraphic nature of the reservoir. Also, initial rates from the primary zones were so high it was not necessary to maximize recovery from lower-porosity and -permeability zones. Finally, during the initial development phase, there were 81 separate operators, with as many as 250 rigs operating simultaneously. All these factors led the initial operators to either bypass or not effectively produce a considerable amount of pay in the SACROC Unit reservoir.
Initially, only a few wells in the field were drilled to the oil/water contact (OWC). Those wells, drilled in the south, rarely penetrated more than 100 ft of the reservoir. During the early 1950's, most wells in the unit were deepened to -4,500 ft. On the basis of drillstem tests and production data, it was determined that the first occurrence of water was approximately -4,500 ft subsea. After deepening the wells, the operators began to suspect that there might be an extensive stratigraphic component to the reservoir. Throughout the reservoir, there are a few dense, tight streaks and thin shale zones. These tight zones were thought to be discontinuous, with the entire reservoir in pressure communication. Later, wells deepened to -4,500 ft found bottomhole pressures (BHP's) much higher than those in the shallower zones. This data indicated vertical isolation of the deeper zones from the shallower zones. Thus, the tight streaks in the reservoir were probably continuous across the field, with fluid flow being essentially horizontal. Even with this information, the reservoir continued to be described as a massive reef buildup by the original operators. Unit geologic studies in the late 1960's and early 1970's improved the reservoir description. These unpublished studies (done by Standard Oil of Texas/Chevron) indicate that numerous stacked, shoaling-upward cycles make up the reservoir. Laterally, the facies within the cycles were found to change abruptly.9 On the basis of this work and electric log correlations, the operators attempted to subdivide the reservoir into five major zones. Because deposition of the Horseshoe Atoll occurred in the middle of a relatively sediment-starved basin, no widespread shale markers were deposited.7 This, coupled with rapid lateral shifting of the depositional facies, made unitwide correlations unreliable. The lack of reliable markers to constrain correlations made it difficult to exploit this stratigraphically complex reservoir properly.
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