Sequential Steam: An Engineered Cyclic Steaming Method

Abstract

"The Potter Sand... has responded well to virtually all thermal recoverymethods... Cyclic steam, however, has been the best EOR process for selectedleases." Summary Cyclic steam injection has been the most widely used EOR method in areas ofthe Potter sand in the Midway-Sunset field, Kern County, CA. This paperdiscusses the field pilot and the statistical and theoretical studies leadingto the design of a sequential steaming process, plus the implementation of thisprocess on three leases. Introduction Oil is produced from many different sands in the Midway-Sunset field, andthermal EOR is the predominant recovery method. The Potter sand. the mostprolific reservoir in the field. has responded well to virtually all thermalrecovery methods. including steamdrive, in-situ combustion. and cyclic steamstimulation. Since the early 1960's, Santa Fe Energy Co. has experimented withall these methods in the Potter sands with varying degrees of success. Cyclicsteam, however. has been the best EOR process for selected leases. Indeveloping our cyclic steam technique. we devised a method called"sequential steam that we feel is a superior way to process these thick, steeply dipping, heavy-oil sands. Geology The Potter sands. the primary producing zones in the northern end of theMidway-Sunset field (Fig. 1), are in the upper portion of the Reef Ridgeformation of Late Miocene Age. According to Lennon, the sands are predominantlya series of fan-channel complexes formed during rapid subsidence of the basin. Coarse granitic debris from the coastal range flowed down marine canyons into adeepwater fan. The depositional character of the Potter is extremely variable, ranging from massive, conglomeritic. debris-flow-filled, narrow channels tothinly bedded, laterally extensive, very-fine-grained, distal-fan turbidites. The subsidence produced a generally transgressive sequence; that is, thedeeper-water. thinly bedded deposits tend to overlie stratigraphically thecoarser, shallow-water channels. Subsequent uplift and tilting produced anerosional surface upon which the Pleistocene Age lacustrine and alluvial-fanTulare formation was deposited. The final uplift and erosional sequence leftthe Potter sands in the setting shown in Fig. 2. The producing sands outcrop atthe western limit of Potter production and are covered by a rapidly thickeningwedge of Tulare silts and sands toward the east. Dips range from about 40 atthe west to 20 at the east and trend north to east. Net sands also are highlyvariable. In the northwestern region of Potter production, especially in the 21Fee lease (see Fig. 1 for all lease locations), where the deposition ispredominantly debris-filled channels. The few existing permeability barriersare discontinuous (presumably caused by channel scour) and often will notcorrelate along strike from one well location to the next. Consequently, netsand counts range up to 700 ft, with as little as 50 ft of shale intervals. Thedeltaic distal-fan deposits to the east are finely interbedded sands and shaleswith individual members ranging in thickness from inches to tens of feet. Netsands may total less than 100 ft in a gross interval of 200 Common reservoir properties in these leases are low-gravity crude (11.5 to13 API). fairly steep dips, good lateral permeability between well locations, low water cuts (~40%). an inactive aquifer. and a reservoir mineralogyapparently highly resistant to damage by steam. History Upstructure Potter Sand Steamflood Pilots. Santa Fe began cyclic steamoperations in the Midway-Sunset field in early 1964. Well No. 36, 27 USL lease, was steamed with a 150-psi tank heating boiler. That first cycle experiencedpeak oil rates of nearly 200 B/D and convinced us that steam was a veryattractive EOR method in this reservoir. The first steamdrive in the area ofinterest began in Aug. 1967. Pilot 201–27 Fee was a single inverted five-spot(see Fig. 1) in an upstructure attic location. Unfortunately. as shown in Fig.3, the project performed poorly, with an instantaneous incremental steam/oilratio (SOR) of more than 9 bbl/bbl [fuel/oil ratio (FOR) of 0.73] during itsbest performance in 1969 and 1970. Cumulative incremental SOR performance wasabout 17 bbl/bbl (FOR of 1.3). Note that once steam decreased to a ratecommensurate with the observed production rate, all response was lost. Apostaudit of this drive showed conclusive evidence of uncontrolled steam lossto the upstructure air zone, which is laterally unbounded for all practicalpurposes. JPT P. 848⁁