Performance of Payoa Basal Eocene Block A Reservoir

Abstract

Abstract The Payoa field Eocene reservoir is a steeply dippingstratigraphic trap with a large vertical oil column overlainby a small gas cap. The trap for oil accumulation wasformed by a major fault on the flank of an antidine, andcross-faulting divided the field into separate reservoirs. A 37" dip of the producing formations allowed 2,700 ft ofoil column to exist between the initial gas-oil and water-oilcontacts. The large vertical height of the oil column relativeto the thin producing section created a variation in fluidcharacteristics from a saturated oil at the gas-oil contactto a highly undersaturated oil near the oil-water contact. Modifications in theoretical methods of reservoirevaluation were necessary to predict reservoir performancebecause of the unique fluid and pressure characteristics. Fully realizing that an innovated approach to reservoirevaluation would be subject to considerable error, it wasconcluded that a gas injection and pressure maintenance programshould be initiated early in the life of the field. Betweenthe time the field was put on production and the initiationof gas injection, the reservoir suffered a decline inproducing rate and a sharp decline in reservoir pressure. Also, the GOR increased quite rapidly. The pressure maintenanceprogram has stabilized the producing rate and arrested thepressure decline. By shutting in the high GOR wells, theGOR was reduced and stabilized at a minimum. Estimatesof reservoir performance show that the pressuremaintenance program will recover 50 percent of the stock-tankbarrels of oil in place as compared with only 15 percentrecovery by primary depletion. Introduction Payoa field, located approximately 200 airline milesnorth of Bogota, Colombia, in the middle MagdalenaRiver valley, is operated by Cities Service Oil Co. for acombine consisting of Colombia-Cities Service PetroleumCorp., Pan American Colombia Oil Co., ColombiaAtlantic Petroleum Co. and Empresa Colombiana dePetroleos. Each company recognized early that the BasalEocene reservoir was a prime prospect for the applicationof reservoir engineering technology. An added incentivewas the possibility of a large, irrevocable loss of oil ifprimary operations were conducted to their economiclimit. Often, in reservoir engineering perspective, neither thedata nor the applicable calculation methods are completelyadequate to describe reservoir performance. To someextent, previous experience must be utilized inconjunction with theoretical applications for each reservoir. In some instances, innovations to accepted theoreticalcalculations are necessary to describe reservoir performance. A variation in reservoir fluid and pressurecharacteristics of the Basal Eocene reservoir presented atwofold engineering problem. First, fluid properties had to bedeveloped that would effectively represent a variation from asaturated oil to a highly undersaturated oil. This data then had tobe related to initial and saturation pressures so that a modifiedapproach to the material balance and fractional flow concepts could beutilized to predict reservoir performance. This article presents thedevelopment of data and its application to reservoir performanceestimates for the Basal Eocene reservoir labeled Block A in Fig. 1. JPT P. 31ˆ