Advanced Decline Curve Analysis Identifies Fracture Stimulation Potential
Authors: Fetkovich E.J. 
Published: December 2007

The following report discusses a decline curve study that was performed on the Phillips State 35 lease located in Lea County, New Mexico and the resulting fracture stimulation opportunity that was identified at both State 35 and the East Vacuum Grayburg San Andres Unit (EVGSAU). This study emphasizes the use of fundamental reservoir engineering techniques to identify differential depletion and an opportunity to increase production and reserves through stimulation of the low permeability layer. Application of the technique known as Decline Curve Analysis to each well in the State 35 lease allowed for the determination of differential depletion occurring between the two major reservoir packages, the Upper and Lower San Andres formations. Stimulations of the tighter, Upper San Andres, layer were performed to improve performance from this layer. Validation of the technology allowed for the work to be continued to EVGSAU. Decline curve concepts, fracture stimulation techniques, and production results will be discussed.

Introduction

Many view oil and gas in West Texas as a generally played out series of fully developed fields. The only remaining potential is that of waterflood or CO2. Although significant opportunity does exist with this technology, there is another technology that presents an opportunity that has not been greatly recognized. This opportunity lies in the recognition of reservoir vertical heterogeneity and its implications to production and ultimate reserves recovery. This vertical heterogeneity is referred to as layered no-crossflow which means that there is no pressure or rate communication anywhere between reservoir layers except at the wellbore (Fig. 1). This layered no-crossflow, differential depletion, behavior becomes important when the layers are depleting at significantly different rates. This situation can be found anytime there is a tight, non productive, layer between productive layers. The non productive layer can be as little as a few feet thick and still provide an effective barrier. Differential depletion can occur between layers because of any one or a combination of permeability, thickness, layer area, porosity, water saturation, pressure, and/or skin. Notice that thickness, layer area, water saturation, and pressure all determine layer volume. Throughout this paper, reference will be made to the high versus low permeability layer. This terminology is used in order to remain consistent with previous works on the subject. Although permeability is the most significant, the discussion is really between the high versus low deliverability layers as evidenced by the number of contributing variables. Recognition of the layer(s) that are not being adequately drained compared to the other layer(s), i.e. differential depletion, is where the opportunity lies. Stimulation of the less productive layer(s) can allow for both increased production rates and reserves. Various techniques can be used to identify layered no-crossflow performance such as measuring the pressure of individual layers or running production logs which would show wellbore backflow during shut-ins or rate reductions.

Decline Curve Analysis can be used to recognize layered no-crossflow production from surface production data. With this method, data captured for allocation and state reporting requirements is plotted either by well, by lease, or by field in a log-log format. From this format and using the Fetkovich type curve, a determination of whether a well is performing under transient or depletion decline can be established. See Ref. 2 for a discussion of type curve matching. If the production data is determined to be in depletion decline, the data will follow a character that can be described by a decline curve exponent b value.

The significance of the decline curve exponent b value is that for single layer performance, the b value will lie between 0 and 0.5. With layered no-crossflow performance, however, the b value can be between 0.5 and 1.0. P. 627^