The Street Ranch Pilot Test of Fracture-Assisted Steamflood Technology

Abstract

Summary A new approach to steamflooding extremely viscous heavy-oil reservoirs and tar sand deposits has been developed and tested successfully in the San Miguel-4 tar sand reservoir located in Maverick County, TX. The process is called fracture-assisted steamflood technology process is called fracture-assisted steamflood technology (FAST), and the first application in a tar sand was performed in a 5-acre (2-ha) inverted five-spot pilot pattern performed in a 5-acre (2-ha) inverted five-spot pilot pattern located on Conoco's Street Ranch lease. During a 31-month period, ending June 1980, the Street Ranch pilot produced 169,040 bbl (26 875 m3) of -2 deg. API pilot produced 169,040 bbl (26 875 m3) of -2 deg. API (1093-kg/m3) tar with monthly tar-production rates occasionally exceeding 300 B/D (48 m3/d). Postpilot core wells indicated residual tar saturations as low as 8% and an average recovery efficiency of better than 50%. Introduction Low injectivity is frequently a problem when steamflooding heavy-oil reservoirs and it is almost always a critical limitation when working with extremely viscous tar sand deposits such as those in Canada, Utah, and Texas. One such resource is a 50-ft- (15-m-) thick layer of partially consolidated sandstone and hydrocarbon known as the South Texas tar sands. It is contained within the San Miguel-4 sand, which is found at a depth of around 1,200 to 2,300 ft (366 to 701 m) under some 90 sq miles (233 km2) of ranchland along the Maverick/Zavala county lines about 30 miles (48.3 km) northeast of Eagle Pass. Current estimates indicate that the San Miguel-4 sand contains between 2 and 3 billion bbl (0.32 to 0.48 km3) of -2 deg. API (1093-kg/m3) gravity tar. Conoco Inc. currently owns or has leased about 29,860 acres (12 084 ha) in the area known as the Saner Ranch field (Fig. 1), and has been working over the last 7 years to develop this particular resource. The unique properties of this resource base have made it necessary to develop new in-situ recovery methods to improve recovery efficiency. San Miguel Tar Sand Deposit Geology The San Miguel-4 is the fourth in a series of nine sands deposited within the Taylor shale sequence of the Montana group. This places deposition during the upper Cretaceous period of the Mesozoic era. The northwesterly and updip location of each successive sand unit indicates that the San Miguel series is a product of an overall marine transgression. The San Miguel-4 deposit is predominantly sandstone with numerous irregular limestone intercalations. Its wide geographical distribution suggests that it represents a highly reworked deltaic deposit on a shallow marine shelf with occasional influence by an upper shelf, barrier bar, strand plain environment. The high degree of reworking was caused mostly by long shore currents during a fairly steady, moderate-energy period as indicated by the very low clay content, good sorting, and the presence of thin-shell pelecypods. A fairly high content presence of thin-shell pelecypods. A fairly high content of feldspars, from 10 to 45 %, indicates short exposure to weathering agents and implies rapid transport and burial. Reservoir Conditions The San Miguel-4 sand outcrops just north of the Maverick/Kinney county lines and dips to the south-southeast at a slope of 2 deg. At the southern edge of Conoco's property (Fig. 1) the formation is about 2,300 ft (701 m) deep. Gross thickness ranges from 20 to 80 ft (6.1 to 24.4 m), with the average close to 50 ft (15.2 m). Except for the interspersed limestone streaks, the sand is very clean, with a mean particle size of 110 microns (110 mu m) and a Schwartz uniformity coefficient of less than two. Its cleanliness and uniformity result in porosities of 26 to 30% and permeabilities of 250 to 1,000 md. JPT P. 511