Acoustic Determination of Producing Bottomhole Pressure

Abstract

Summary. This paper discusses the acoustic determination of producing bottomhole pressure (BHP). Two different techniques are presented for wells that have liquid above the formation and gas flowing upward through the gaseous liquid column. One technique involves the acoustic measurement of the liquid level and the casing-pressure buildup rate when the casinghead valve is closed. When these data are used along with an empirically derived correlation given here, the gradient of the gaseous liquid column in the annulus can be obtained. This technique offers a reasonably accurate procedure for determining the producing BHP of a well by acoustic means. The second method involves two acoustic measurements. A backpressure valve is used in the casing head to depress and to stabilize the liquid level at two positions while the well is produced at a constant rate. The gradient of the gaseous liquid column is produced at a constant rate. The gradient of the gaseous liquid column is then calculated and extrapolated to the formation depth. This paper discusses results from the field testing of numerous wells where the actual gradients of gaseous liquid columns were measured in a variety of casing/tubing sizes, oil gravities, gas flow rates, and pressures. pressures. Introduction The producing-rate efficiency of a well can be determined with the curve of inflow performance relationship, which requires knowledge of the producing and static BHP'S. Techniques for determination of static BHP's by acoustic means have been presented in Refs. 2 through 5. These techniques have proved to be sufficiently accurate for most conditions. The producing BHP is the sum of the surface casing pressure plus the pressure from the column of fluids in the annulus. plus the pressure from the column of fluids in the annulus. The fluid distribution in the annulus is a function of the introducing conditions of the particular well. Three situations are generally found in the field:the liquid level is at or near the formation and casinghead gas may or may not produced;the liquid level is above the formation and casinghead gas is not produced; andthe liquid level is above the formation and casinghead gas is produced. produced. Fig. 1 illustrates these three cases. For Cases A and B, the pressure distribution is well defined from a measurement of the pressure distribution is well defined from a measurement of the pressure at the surface, a knowledge of the properties of the fluids, pressure at the surface, a knowledge of the properties of the fluids, and the position of the liquid level. Case C, on the other hand, involves the uncertainty of the gaseous liquid column gradient as a result of the annular gas flow. Liquid Level at Formation (Case A). The casinghead pressure constitutes the major portion of the producing BHP in normal-depth wells because the pressure from the gas column is relatively small. Even when gas is being vented, the frictional pressure losses are minimal. BHP calculation is undertaken from a measurement of the casinghead pressure, the knowledge of the gas composition, and the temperature distribution as described in Ref. 2. Or the BHP calculation can be performed by a computer program given in Ref. 6. This program also includes Cases B and C. The liquid level will always be at the tubing perforations when a well is being produced with the casing valves closed and free gas is flowing from the formation. Liquid Level Above Formation Without Free Gas Flow From Reservoir (Case B). At stabilized producing conditions, the liquid above the tubing perforations is 100% oil. This producing BHP is calculated from measurement of the surface casinghead pressure, measurement of the depth to the liquid level by an acoustic survey, and a knowledge of the oil and gas properties. Details of the calculation are given in Ref. 2. Liquid level Above Formation With Casinghead Gas Flow (Case C). This condition results in a gaseous annular liquid column. At stabilized producing conditions, the oil in the casing annulus becomes saturated with the gas that is continuously flowing to the surface. Consequently, if gas is being vented at the surface at a constant rate, free gas is being produced from the formation simultaneously with the oil. Generally, most oil is produced througthe pump while most free gas is produced up the casing annulus. pump while most free gas is produced up the casing annulus. BHP calculation is undertaken from a measurement of casinghead pressure, knowledge of oil and gas properties, and an estimate of pressure, knowledge of oil and gas properties, and an estimate of the oil fraction in the annular liquid. The fraction estimate is required to obtain the gradient of the gas/liquid mixture. This problem has received considerable attention by numerous authors. These techniques involve the determination of the gas flow rate up the annulus and, in turn, the calculation of the amount of liquid present in the gaseous liquid column by use of such well conditions as casing/tubing sizes, liquid properties, and pressure. All these methods are based on a combination of theoretical and empirical models and yield different results for a given set of conditions as shown by Kabir and Hasan. Because of the disagreement of these techniques, a comprehensive field study was performed to determine directly the gradient of gaseous liquid columns. The wells tested during this study included casing sizes from 4.5 to 7 in. [11.4 to 18 cm] and oil gravities between 32 and 43 deg. API [0.86 and 0.81 g/cm3]. Long gaseous liquid columns of more than 5,000 ft [1525 m] were studied in wells up to 9,000 ft [2745 m] deep. Annular gas flow rates ranged from 13 to 120 Mcf/D [368 to 3400 m3/d] and oil fractions ranging from 20 to 77% were measured. The wells were located in regions of normal temperature gradients in the range of 0.9 to 1.2deg.F/100 ft [16.4 to 21.9 mK/m]. When a gaseous liquid column exists in the annulus of a well producing at stabilized conditions. the pressure at any depth in the producing at stabilized conditions. the pressure at any depth in the gaseous column is independent of the surface pressure. This is il-lustrated in Fig. 2, which is a schematic of a well producing at three different values of casinghead pressure. The producing BHP remains unaffected by the changes in surface pressure and liquid level as long as the production rates through the tubing and the casing annulus remain constant. The annular pressure for three cases is plotted as a function of depth in Fig. 3. The gradient of the gaseous column (gas/oil mixture) can be obtained from the change in liquid level and in pressure at the gas/liquid interface. P. 617