Case History of Pressure Transient Testing at McElmo Creek, Utah

Abstract

Abstract This paper presents the results of a review of over eight hundred historical pressure tests conducted in the McElmo Creek Unit of southeast Utah. Three hundred pressure transient analyses were performed. The paper describes how this data base was used to characterize trends in pressure, reservoir quality and stimulation effectiveness. The various techniques used to acquire the pressure data are also discussed. Introduction Reservoir pressure testing is conducted throughout the producing life of most oil and gas fields. The testing history for McElmo Creek is shown in Fig. 1. The periods of high test frequency correspond to important stages in the field's development. Initially, pressure data was acquired to monitor primary depletion prior to waterflooding. When waterflooding was initiated in 1962, tests were conducted to assess the pressure support from the water injection. In the early and mid 1980s, pressure data was acquired in advance of the carbon dioxide (CO,) miscible flood (initiated in 1985) to ensure that the reservoir pressure was in excess of the minimum miscibility pressure. By the 1980s, it was recognized that pressure transient testing is valuable to reservoir characterization, and as a result, more pressure transient tests (as opposed to static pressure tests) were conducted. The database of McElmo Creek pressure test data includes some eight hundred and fifty individual tests. Approximately three hundred of these tests include a complete pressure transient suitable for analysis. The large number of tests permits comparison of parameters both within a single well over time and between surrounding waits. This paper provides details on the testing procedures and describes how the test data was used to characterize trends in pressure, reservoir quality and simulation effectiveness. The McElmo Creek Unit, operated by Mobil in the Aneth field of San Juan County, Utah, includes 312 wells of which 119 are injectors. The Unit has been waterflooded since 1962 and has been under a partial CO2 flood since 1985. This field produces oil from the Desert Creek and Lower Ismay zones of the Paradox formation, which is of Pennsylvanian Age. A typical log, showing porosity and core permeability, is presented in Fig. 2. Detailed geologic and fluid property data are available in the literature. PRESSURE DATA ACQUISITION Most of the pressure transient data was acquired through injection falloff tests. Approximately half of these pressures were recorded using downhole gauges. The other half of the falloff tests recorded the tubing pressure at the wellhead and relied upon a pressure extrapolation to get bottomhole pressure. Approximately one-quarter of the pressure transient data was acquired through pressure buildup testing on producing wells. Most of these tests were performed early in the field's life when wells were capable of flowing. Only a few drawdown tests were ever attempted. In general, analysis of the pressure transient data for McElmo Creek wells is straightforward. Those tests which cannot be analyzed adequately with a homogeneous, radial flow model can usually be modeled with a high conductivity fracture model. None of the tests analyzed show the characteristics of a dual porosity (naturally fractured) reservoir. All injection at McElmo Creek is carried out below the formation parting pressure. Dynamic fracture characteristics are therefore not a concern with pressure transient analysis. CONVENTIONAL PRESSURE TRANSIENT TESTING Most of the pressure tests run at McElmo Creek, whether simple static pressure tests or full pressure transient tests, rely on the use of downhole pressure devices. These devices have the benefit of accurately measuring true bottomhole pressure without the need to extrapolate pressure from the surface. P. 359^