Adverse Effects of Water Injection Into Thick Sand Reservoirs Containing Thin, Highly Permeable Lenses

Abstract

Abstract Engineering studies for pressure maintenance by water injection were completed in 1956on four reservoirs located in the Ovejafield of Eastern Venezuela. Injection into the reservoirs began in Nov., 1957. Severechanneling of injected water occurred in the reservoirs, causing the suspension of water injection. Channeling is attributedto selective encroachment of the injectedwater through thin, highly-permeable sandsections. The J3 sand, OM–100 reservoir, is presented as a case history of the existing problem. Preliminary reservoir data indicatedgood sweep efficiencies and high recoveryby water injection. Flood studies on coresamples yielded recoveries in the 45 to60 per cent range. Predicted recoveries by pressure depletion and pressure maintenancewere 10.5 and 18.1 per cent, respectively. Water injection into the J3 sand wasdiscontinued in Nov., 1960, when it becameapparent that continued injection would be detrimental to ultimate recovery. The volumetric size of the water-encroachedpattern was 30 per cent of total reservoir volume. Channeled water extended into updipwells, and continued injection would extend these channels in an updip direction. The remedial approach to depletionis planned for two phases:wet wells will be produced at increased rates to prevent extension of water encroachment; andgas will be injected into the secondarygas cap for pressure maintenance. Preliminary studies indicate an ultimate recovery of 26 million bbl by gas injection, which represents 14 percent of the oil originally in place. Introduction The Oveja field is located in the southernportion of the Greater Oficina area of Eastern Venezuela. The field was discovered in 1952 with the drilling of a wildcat well, Mundo–1, which has been renamed OM–100. Subsequently, field development included88 wells for drainage of the I J3, L1L andL4 sands. Water encroachment through thin sections of highly permeable sand contained in thicksand deposits has yielded adverse effectsto pressure maintenance by water injectionin all of the afore-mentioned sands. Injection of water into the J3, L1L (southern reservoir)and L4 sands has been discontinued. A fourthinjection project, L1L sand (northern reservoir), is currently being studied for possible discontinuation. Water injection into theI2L–3 sand was never initiated because ofobserved adverse effects in the other reservoirs. The problem of water encroachment, as displayed in the J3 sand, will be considered in this paper. The J3 sand is selected for presentation because the encroachment pattern is more severe than in the other sands. Geological and Reservoir Data Structure The Oveja sand deposits are channel deposits with closure to the south formed by the main Oveja fault. The structureand isopach maps presented in Figs. 1 and2 disclose a 20 dip to the northeast andsand pinch-outs occurring to the east and west. Reservoir boundaries are completedto the north by a southwest-to-northeastmajor fault and the water-oil contact at3,438-ft subsea. Size Surface area overlying the J3 sandcovers 2,216 acres and the average net sandthickness is 44 ft, giving a total reservoir volume of 97,500 acre-ft. Volumetric calculations indicate that the pool originally contained185 million STB of oil, a volume which hasbeen confirmed within a small percentage by material-balance calculations. JPT P. 253^