Effects of Underbalance on Perforation Flow

Abstract

Summary In a study of perforation flow in standard API test targets, underbalance between 500 and 1,000 psi [3.4 and 6.9 MPa] was needed to obtain optimum flow efficiency. Perforation cleanup due to transient pressure gradients was separated from that caused by postshot, steady-state flow. Perforations with low initial flow efficiencies could be improved by steady-state washing, but not to optimum levels. The amount of rock debris washed from the perforation during the washing phase correlated directly with the shooting underbalance applied. Tests performed in kerosene-saturated, brine-free targets resulted in generally lower flow efficiencies and required higher underbalances to obtain optimum levels. Introduction Restricted flow in perforations created by jet penetrators can adversely affect productivity in natural well completions. In particular, reduced flow can be caused by plugging of the perforation with rock or charge debris or by mechanical damage to the rock matrix from high shock pressures. Klotz et al., Locke, and later Tariq calculated the effect of reduced permeability in a crushed zone surrounding the perforation on well productivity. Use of techniques for completing wells with underbalanced fluid pressure conditions is now common, both to prevent formation damage by completion fluids and to clean out the perforations. Proper design of such completions requires an estimate of the underbalance needed to obtain maximum productivity. Using early laboratory experiments, Bell et al. suggested that 200-psi [1.4-MPa] underbalance was sufficient to obtain maximum perforation flow in Berea sandstone targets, and standard API flow testing procedures use this value. King et al. used well-productivity data to identify the needed level of underbalance and determined the adequacy of a given underbalance by whether subsequent acidizing treatment improved well productivity. The data, presented as a function of matrix permeability, suggest that underbalance in excess of 500 psi [3.4 MPa] may be needed to obtain optimum productivity in sandstones similar to Berea. In the absence of damage by completion fluids, cleanup of a perforation occurs through flushing of loose debris and removal or crushed, reduced-permeability rock from its walls. In studies using radial-flow targets at 500- and 1,000-psi [3.4- and 6.9-MPa] underbalance, Regalbuto and Riggs showed that flow efficiency correlates with increased volume of the perforation. They related this increased volume to removal of "crushed" material and showed that immediate or delayed surging produces similar results. Underbalanced surging may remove damaged rock in two way, first through high transient fluid pressure gradients and flow rates and second by steady-state pressure gradients and flow across the zone of reduced permeability. The former lasts only a short time and involves limited flow volumes; the latter occurs over an extended period of flow. To gain a better understanding of how transient and steady-state underbalances affect perforation flow efficiency and to determine better the underbalance needed to obtain maximum efficiency, we performed a series of experiments, measuring flow efficiency as a function of underbalance. The procedure was designed to separate transient surge effects from those caused by steady-state flow. To concentrate on mechanical flow restrictions, standard API procedures were modified to eliminate two-phase flow and temperature complications. Methods and Procedure The apparatus (Fig. 1) consisted of a modified API RP 43 cannister attached directly to a small pressure vessel (the well vessel) that simulates a fluid-filled wellbore. This arrangement allows control of contaminants and completion-fluid composition and eliminates the need for a large test well. The vessel contains a specially designed cylindrical charge carrier, and a small accumulator is attached to supply fluid to maintain the desired wellbore pressure after the shot is fired. All tests were carried out with a conventional jet penetrator with 6.5 g of explosive. Cores were prepared by standard API RP 43 Sec. 11 procedures (i.e., saturated with brine and flushed with kerosene to constant effective permeability) and with a brine-free, kerosene-saturation procedure. These simplified brine-free tests were included to avoid complications caused by two-phase fluid flow and adsorption/desorption reactions between brine and rock. The well vessel was filled with kerosene completion fluid for brinefree targets or with brine for standard API targets. Well pressure was 1,000 psi [6.9 MPa] for all tests, and core pressure was varied between 1,000 and 3,000 psi [6.9 and 20.7 MPa] to achieve underbalances between 0 and 2,000 psi [O and 13.8 MPa]. All experiments were done at ambient temperature to avoid any changes in permeability with temperature. Postshot flow measurements were made with differential pressures between 200 and 800 psi [1.4 and 5.5 MPa] with ambient well pressure. This procedure eliminated the need to flow grit-bearing fluids through backpressure regulators and ensured constant differential pressure. The first step in each test was saturating cores with either brine or kerosene and flow-testing in a Hassler permeameter as described in API RP 43, Sec. 11. In both cases, flow was continued until no further change in permeability was observed. Absolute permeabilities were approximately 300 md and core porosities were 19 to 20%. Each core was then inserted in a modified test cannister and grouted with Hydromite. After a minimum 5-day cure time, testing began by assembling gun, well vessel, and cannister, filling the well vessel with kerosene or brine and pressurizing both vessel and core to 1,000 psi [6.9 MPa]. Core pressure was then increased to obtain the desired shooting underbalance and all valves were closed. After the shot was fired, the core and well vessel were vented simultaneously to avoid flow through the core. To obtain uncontaminated washings from the core, the well vessel and accumulator, which contained most of the charge debris, were removed from the cannister. With the perforation exposed to ambient pressure, the core pressure was then raised to obtain the 200-psi [ 1.4-MPa] flow differential specified in RP 43. Flowrate measurements began immediately, and the rate recorded during the first I or 2 L is reported below as the initial rate. Flow continued under these conditions until a constant rate was achieved, or to a minimum of 20 L. Differential pressure was then increased in steps to 400, 600, and 700 to 800 psi [2.8, 4. 1, and 4.8 to 5.5 MPa], the highest pressure being governed by the pump's capacity. At each pressure level, flow was maintained for 5 to 10 L, usually long enough to obtain a constant flow rate. In selected tests, fluids from the perforation were filtered to collect any solids washed from the core. These were observed qualitatively and total weight was recorded. After completion of flow measurements, the perforations were blown out and total core penetration was measured as described in RP 43. These data, combined with the absolute or effective permeabilities measured earlier, were used to convert flow rates to core flow efficiencies (CFE's). This parameter, defined in RP 43, provides a measure of the flow rate corrected for both matrix permeability and perforation length. SPEPE P. 113^