Field Observations of Perforation Inflow Diagnostic (PID) Testing of Shallow Low-Permeability Gas Wells

Abstract

Abstract In this paper, we present case studies of closed chamber testing of low-permeability gas wells immediately after perforating and report the observations made from the interpretation of these tests. We refer to this type of testing as Perforation Inflow Diagnostic (PID) testing. The procedure for PID testing is:to remove liquids and gases from the wellbore in order to create underbalanced conditions before perforating, andto record surface and/or subsurface pressures in a closed chamber environment continuously throughout the fill up period of the wellbore. The analysis process is to derive a simultaneous gas inflow rate immediately after perforating using the closed chamber method to determine PID absolute open flow (AOF) potential, in situ gas permeability, skin and reservoir pressure. The assumptions of the diagnostic technique include isothermal and homogeneous reservoir conditions, single phase gas inflow and constant gas chamber volume. In a majority of the cases where we applied this procedure, we observed an ideal laminar inflow profile. However, a number of pressure responses showed a deviation from this idealized trend suggesting possible inflow of drilling mud filtrates or connate water inflow associated with the gas. In this paper, we present the well publicized backpressure plot as a diagnostic plot to help identify liquid inflow when only the simplest of surface recorders are utilized for field data measurements. Introduction Perforation Inflow Diagnostic (PID) tests, Impulse tests, Slug tests and Closed Chamber tests are all similar in methodology. By whatever name, they can provide estimates of maximum gas inflow (AOF) capability, flow capacity (kh), skin factor (S) and initial reservoir pressure (pi). The perforation inflow diagnostic (PID) testing analysis method is an extension (and specialization) of the closed chamber drillstem testing (DST) methods developed in the 1970s by Lloyd Alexander(1). Field examples of this method in tight gas reservoirs together with typical diagnostic surface pressure plots were presented by Reid(2) in 1981. This method of closed chamber testing has proven to be a very effective procedure when applied to PID testing. The method is specifically applied to gas wells and is used to analyze the period subsequently after perforating a zone of interest(3). Since the wellhead is shut in during the test, this procedure presents itself as a safe, cost effective and environmentally friendly technique that can be performed using surface pressure recorders. The purpose of this paper is to address the question, "Does the pressure response subsequent to perforating provide meaningful reservoir information? " The method involves using standard perforating equipment and operations. The technology involves conditioning the wellbore for maximum underbalanced perforating conditions and then monitoring the surface and/or subsurface pressure response. Unlike conventional testing procedures, the surface valve is closed during the entire flow period and the formation fluids are produced into the closed chamber (casing and/or tubing volume). The pressure response measured after perforating is converted to gas inflow rates using established closed chamber calculations. This pressure and rate data can then be applied to transient flow theory for well and reservoir evaluations.