Correlating petrophysical and flood performance in the Levelland slaughter field
Watson, Marshall C.
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The Levelland and Slaughter fields combined have produced over 1.6 billion bbls from 1937 to date from 6000 wells and currently produce 6% of the oil in Texas. Most of the field is under water and CO2 flood operations. This project investigates reservoir and petrophysical characteristics of various areas in the Levelland Slaughter field in order to assess relation to performance of secondary and tertiary recovery. The benefits would be to use this relationship to identify depositional environment/facies where little to no core data exists. In areas where no flood has been installed, the relationships developed herein could assist in the ability to predict flood recovery and the method of development. The Levelland Slaughter field is similar to several other ramp type, carbonate fields in the Permian Basin. Results once applied and proved successful in the Levelland Slaughter field not only could be applied to many other fields in the Permian Basin, but also to similar oil reservoirs all over the world. The first objective was to divide the field into areas of like depositional environments. This entailed identifying depositional environments via log, core and production analysis. The objective is to integrate geology and production into the study to ascertain whether like data can be considered, especially in an east west sequence across Levelland Slaughter. There are three depositional environments at Levelland Slaughter as follows; shelf with shoals, lagoonal and intertidal/near shore. Once subdivided, relationships between petrophysical properties and secondary recovery rates are developed utilizing Lucia’s rock fabric classification and production plot methods demonstrated by Reza Fassihi of BP. Fassihi demonstrated the method of plotting water cut against fractional secondary oil recovered enabled one to derive the matrix/fracture flow and storage capacity relationships. Based on curves developed by Fassihi, one can conclude there is little to no natural fracturing in the reservoirs in the Levelland Slaughter field. Lucia demonstrated that by plotting porosity against permeability in carbonate reservoirs, one could derive the type of rock fabric and detect facies changes. Net pay for primary and secondary recovery can be different and are dependent primarily on permeability and water saturation. Water saturation varies within the pay zone in lower permeability reservoir rock. Below a given permeability, water saturation increases and become movable. In consideration of the fore going, a typical Levelland well in the 1950’s produced water free, but later, prior to waterflooding, produces 20 to 50% water cut. For secondary waterflood recovery considerations, a critical water saturation exist where an oil bank does not develop, thus resulting in prolong recovery periods or little to no recovery from that particular reservoir rock. From the Lucia classification and Fassihi plots, it appears that most of the rock fabrics are similar and only differ in permeability. This is possibly due to anhydrite inclusion that was deposited in the more permeable rock, thus leaving the lower permeability mudstone porosity intact. Some localized areas could have improved reservoir due to subtle changes in elevation. These elevation changes are critical in the western area because small sea level changes caused substantial areas to go from subtidal to intertidal to supratidal/mud flats. Each of the fore mentioned steps resulting in a reduction in reservoir quality due to salt precipitation or anhydrite inclusions. This does not apply to shoal areas, located at shelf margins, because they were never supratidal, thus there are no evaporates. Thus the higher permeability rock demonstrates higher primary recoveries as well as much greater Secondary to Primary ratios (S:P).