Abstract

Single shot laboratory flow tests were performed on perforated Berea and Gold sandstone cores to determine the additional skin caused by the presence of a perforation damage zone. A variety of underbalances, presence of a perforation damage zone. A variety of underbalances, stresses and wellbore volumes were used.

Whole core diametral and axial permeabilities were measured for each core tested. Cores were then perforated under simulated downhole conditions, and radial flow tests were conducted to determine the skin due to the perforation damage zone. Because these measurements gave no information on the distribution of permeability damage, further experiments were carried out using a radial flow permeameter to directly measure the permeability distribution.

The radial flow permeameter confirmed the presence of a lowpermeability region around, but not always adjacent to the perforation permeability region around, but not always adjacent to the perforation tunnel. Permeability damage at the outside of the cores due to the coring process was also determined. process was also determined. Although one set of experiments shot under identical conditions, at a lowunderbalance, showed a degree of randomness in the results, the following observations could be made:

  1. For the conditions tested. there is no apparent stress effect on the perforation damage skin,

  2. The size of accumulator attached to the wellbore affected the resulting skins, and

  3. Results of single shot experiments should be used with extreme care.

Introduction

A number of well performance computer models require input for both wellbore and perforation damage to calculate productivity ratio. For both cases, the damage depth and permeability reduction is required. For perforation damage, the literature suggests 0.5-in. for the depth and a perforation damage, the literature suggests 0.5-in. for the depth and a 0.1 to 0.2 permeability reduction factor based on core flow efficiency(CFE) values from the API RP43 4th Edition Section 2 tests. With the increased utilization of tubing-conveyed perforating (TCP) and corresponding higher underbalances, a more definitive answer is required with respect to perforation damage versus rock properties and underbalance levels.

API RP43 5th Edition, published January 1991, includes a new Section 4single shot flow test in Berea sandstone under simulated downhole conditions. The experimental results presented in this and two companion papers are the first set of extensive flow experiments published using papers are the first set of extensive flow experiments published using the basic procedures of the Section 4 flow test to determine perforation damage skin.

Experimental Procedure

The accuracy of an experimentally determined perforation damage skin is directly proportional to the accuracy of the rock permeabilities. Procedures were developed to determine whole core axial and Procedures were developed to determine whole core axial and diametral permeabilities under high hydrostatic stress with high flowrates. Material compatibility, selection and filtration of pure kerosene, filter size requirements and system pressure drop versus flow rate were just a few of the considerations. Appendix A reviews some of the permeability data obtained by various procedures. The data suggest that permeability data obtained by various procedures. The data suggest that determination of whole core permeabilities better than +/-10% is unrealistic.

The outcrop cores were oven dried and vacuum saturated with 2% NaCl brine. Clean kerosene was flowed through the core until no brine waseluted and steady-state flow obtained. Whole core permeability was obtained at this point. The core was placed under hydrostatic pressure(Fig. 1). representing overburden stress, pore and wellbore pressures were adjusted and the perforator shot. The wellbore was attached to an air-over-liquid accumulator to simulate a closed wellbore with a gas cap.

During shooting, the core was isolated from any external fluid input. Thus, after shooting, the pore and wellbore pressures equilibrated to the original wellbore pressure. The wellbore, and thus pore pressures are reduced to atmospheric, and the overburden pressure was reduced to maintain the original effective stress. The perforation was then flowed at as pecified pressure differential until steady-state flow was reached. The flowing differential pressure may be increased to determine cleanup versus"postshot" pressure differential.

Two aspects of the above experimental procedure limit the simulation of downhole fluid dynamics. First, due to the limited core volume, the only fluid that flows through the perforation, prior to any postshot flow, is that which is compressed in the rock pores at the initial differential pressure between the wellbore and the core. Thus, the total laboratory flow pressure between the wellbore and the core. Thus, the total laboratory flow is less than that downhole for equal conditions.

P. 491

This content is only available via PDF.
You can access this article if you purchase or spend a download.