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Since the beginning of injection in 1972, carbon dioxide has been utilized at the SACROC Unit of the Kelly-Snyder Field to improve and enhance oil recovery. As one of the first fields in the world to attempt this technique, SACROC provides a unique opportunity to study, learn from, and improve upon the development of this technology.
Between initial gas injection and the year 2000, approximately 1 TCF of CO2 had been injected into the Canyon Reef reservoir. While multiple pilot programs from this time period reported incremental response following CO2 injection, many of these volumes are now attributed to residual waterflood affects as many of these projects were not truly miscible CO2 floods. Since that time, advances in technology, improvements in reservoir knowledge, and increases in scale have expanded the boundaries of economically viable enhanced oil recovery. As a result, projects that were once considered marginal in reservoir quality and redevelopment potential are now exhibiting improved recovery responses across the field. Since taking over as operator at SACROC in 2000, Kinder Morgan has redeveloped more than 25 major project areas including more than 600 patterns within the unit. Under Kinder Morgan operations, cumulative CO2 injection has surpassed 7 TCF yielding a cumulative enhanced oil recovery of over 180 million barrels.
This case study will review the history of enhanced oil recovery at SACROC, discuss changes in theory over time, and provide a forward look at what remains for this field. This analysis will focus on the first CO2 injection pilot programs, dimensionless oil and CO2 recovery performance and their impact on forecasting, and inclusion of the transition zone and other marginal pay areas in redevelopment. Additionally, this study will highlight the major operational issues encountered over time and the associated improvements that have contributed to enhanced oil recovery. These advancements include conformance improvements, upgraded field surveillance, and proper management of the overall injection- production system. In conclusion, the case will review the Hawaii and East Flank CO2 Flood Expansions: two recently developed marginal pay projects undergoing miscible CO2 flooding at SACROC. Successful redevelopment of these marginal pay projects is allowing for expansion to areas of the field once considered marginal in quality and accordingly uneconomic.
Hawkins, J.T. (Pennzoil E&P Co. ) | Benvegnu, A.J. (Pennzoil E&P Co. ) | Wingate, T.P. (Pennzoil E&P Co. ) | McKamie, J.D. (Pennzoil E&P Co. ) | Pickard, C.D. (Pennzoil E&P Co. ) | Altum, J.T. (Pennzoil E&P Co. )
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.
Abstract This paper presents an overview of the SACROC Unit's activity focusing on different CO2 injection and WAG projects that have made the SACROC Unit one of the most successful CO2 injection projects in the world. The main objective of this work was to review CO2 injection and injection rate losses due to the CO2 /WAG miscible displacement process in the SACROC Unit and recommend an injection strategy for WAG-sensitive patterns. Two types of pattern CO2 /WAG injection rate performance were observed, 1) WAG-sensitive and 2) WAG insensitive. WAG-sensitive patterns displayed loss of CO2 injectivity, exceeding 80% in some patterns, during water-alternating-gas (WAG) injection, and an apparent reduction in water injectivity during the follow-up brine injection. This injectivity loss was observed in over 150 injection patterns. Over time, CO2 injectivity tended to return to prior-to-WAG values. WAG-insensitive patterns suffer from these injectivity losses and were characterized by differences in 1) injectivity profiles, 2) Dykstra-Parsons coefficients, and 3) injectivity indexes. In the majority of WAG-sensitive patterns, injectivity profiles redistributed after CO2 injection, while WAG-insensitive patterns did not show a significant change in their injectivity profiles over time. In a limited data set, the mean Dykstra-Parsons coefficient calculated for WAG-sensitive patterns was 0.83, while for WAG-insensitive patterns the mean Dykstra-Parsons coefficient was 0.76. However it was observed that in the lower Dykstra-Parsons patterns (WAG-insensitive patterns) much larger injectivity indexes were also observed; 19.5 bbl/day/psi, compared to 8.5 bbl/day/psi for higher Dykstra-Parsons patterns. This suggests that the WAG-insensitive patterns were dominated by fracture flow rather than matrix flow. These observations indicate that the WAG injection process in these heterogeneous SACROC wells is successful in diverting the injected fluids from zones with higher permeability to zones with lower permeability. For wells with injectivity values of less than 10 bbl/day/psi it is recommended to begin CO2 /WAG injection with a long CO2 cycle since they are likely to show sensitivity to WAG. A simulated 5-spot pattern was used to study the injection schedule for WAG-sensitive patterns. Longer CO2 cycles and shorter water cycles improved the injectivity and pattern production. Most importantly, it was observed that increasing producing BHP to MMP resulted in significantly lower GOR.
This paper presents an overview of the SACROC Unit's activity focusing on different carbon dioxide (CO2) injection and water-alternating-gas (WAG) projects that have made the SACROC unit one of the most successful CO2 injection projects in the world. The main objectives of this were was to review CO2 injection and injection-rate losses with respect to the CO2/WAG miscible displacement process in the SACROC Unit and recommend an injection strategy for WAG-sensitive patterns. The Kelly-Snyder field is the largest of a chain of fields along the Pennsylvanian Horseshoe atoll in the Midland Basin. Within this field, the Scurry Area Canyon Reef Operators Committee (SACROC) Unit covers approximately 56,000 acres with 2,800 million STB of original oil in place (OOIP). Limestone is the dominant mineral within the Canyon Reef formation, and less than 3% of the formation exists as thin sections of shale (1–10 ft in thickness) that are important stratigraphic markers. The formation is divided into four major zones: the Cisco, the Green Zone (GZ), the Upper Middle Canyon, and the Lower Middle Canyon (LMC).
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 179569, “Overview of CO2 Injection and WAG Sensitivity in SACROC,” by Reza Barati Ghahfarokhi, SPE, The University of Kansas, and Steve Pennell, Michael Matson, SPE, and Mark Linroth, SPE, Kinder Morgan, prepared for the 2016 SPE Improved Oil Recovery Conference, Tulsa, 11–13 April. The paper has not been peer reviewed.
This paper presents an overview of the SACROC Unit’s activity focusing on different carbon dioxide (CO2) injection and water-alternating-gas (WAG) projects that have made the SACROC unit one of the most successful CO2 injection projects in the world. The main objectives of this were was to review CO2 injection and injection-rate losses with respect to the CO2/WAG miscible displacement process in the SACROC Unit and recommend an injection strategy for WAG-sensitive patterns.
The Kelly-Snyder field is the largest of a chain of fields along the Pennsylvanian Horseshoe atoll in the Midland Basin. Within this field, the Scurry Area Canyon Reef Operators Committee (SACROC) Unit covers approximately 56,000 acres with 2,800 million STB of original oil in place (OOIP). Limestone is the dominant mineral within the Canyon Reef formation, and less than 3% of the formation exists as thin sections of shale (1–10 ft in thickness) that are important stratigraphic markers. The formation is divided into four major zones: the Cisco, the Green Zone (GZ), the Upper Middle Canyon, and the Lower Middle Canyon (LMC). Of these, the GZ shows the highest matrix permeability, significant nonmatrix-flow features, and high-conductivity channels. Moreover, the transition zone (TZ) below the oil/water contact has recently been developed in parts of the SACROC Unit.
Primary Production. Completed in November 1948, the Standard No. 1 Jessie Brown 2 was drilled to 6,700 ft and produced 530 B/D from the Canyon Reef Formation. Located 9 miles northwest of Snyder, Texas, this well was the discovery well of the North Snyder field. The Texas Railroad Commission eventually merged this field with the neighboring Kelly field upon recognizing that both fields produce from the same reservoir. The reservoir thickness varies from 10 ft on the flanks to 900 ft on the crest of the reef. A map of the unit’s 3D structure and thickness is shown in Fig. 1. By late 1950, 1,600 production wells had been drilled on the Kelly- Snyder field on irregular 40-acre spacing.