Underbalanced Perforation Characteristics As Affected By Differential Pressure (1986)

ABSTRACT Underbalanced, tubing conveyed completions are increasing due to the apparent advantages of low debris perforations, reduced completion times and stimulation treatment. This paper extends the work reported previously involving gun perforations made under high differential pressures and radial flow conditions. Radial flow Berea sandstone core specimens are used to determine the perforation characteristics resulting from time dependent pressure differentials between core pressure and "well bore" pressure during the completion process. The primary perforation characteristic studied (Radial Flow Ratio) is defined as the ratio of the perforated flow rate to the flow rate of the unperforated core. The perforated flow rate is measured after the perforation has been made under various time-dependent pressure differentials. The series of flow tests included pressure differentials of 5O0 psi overbalanced, balanced, 500 psi underbalanced and 1000 psi underbalanced. In addition the perforations were surged immediately or subsequent to perforating. The Radial Flow Ratio-(RFR} was affected most by the 500 psi and 1000 psi underbalance. The surged RFR's were from 50% to 58% greater than the no surge RFR's. Perforation hole volumes were increased by as much as 55% by the surge, indicating some elimination of the compacted, damaged zone material. Underbalanced surge of 1000 psi increased hole volume to four times that resulting from 500 psi overbalanced, no surge conditions. Underbalancing and surging appears to improve the flow characteristics of shaped charge perforations by simultaneously reducing the thickness of the low permeability crushed zone and increasing the perforation surface area. INTRODUCTION It is generally, but not universally, held that underbalanced completions result in more productive wells (refs. 1–4). Recent studies of completions made in the Beatrice and Kuparuk River fields are diametrically opposed in their assessment of the efficacy of underbalanced completions (refs. 5, 6). This work is intended to examine the current rationale: "The differential pressure from formation to wellbore results in a "cleansing" flow which displaces debris or compacted material from the perforation". To quantify the effectiveness of a given perforator or perforating technique it is desireable to simulate the real downhole conditions in the laboratory. The current method of perforator performance evaluation (API RP 43) utilizes axial flow through a Berea sandstone core (see Figure 1). The rate of flow of kerosene through the core, before and after perforation, is considered to be a measure of the perforator's flow efficiency (ref.7). This laboratory procedure, based on axial flow, lacks the omnidirectional characteristics of flow into a real perforation. In order to realize the full effects of an underbalanced perforation, it was considered necessary to subject the perforation to radial as well as axial flow conditions. The laboratory target therefore was configured to allow flow into the cylindrical surface of the Berea core, as well as the end. This was done by surrounding the core with a highly permeable synthetic rock such that the pressurized kerosene could access the entire surface of the core except for the face which was to be perforated.