Determination of Well Spacing for Maximum Efficiency in Fluvial Gas Reservoirs Using Geostatistical Evaluation and Forward Stochastic Modeling
Authors: Levey R.A. , Sippel M.A. 
Published: December 2007

Abstract
The Secondary Natural Gas Recovery: Targeted Technology Applications for Infield Reserve Growth of Gulf Coast Reservoirs Program was a joint-venture research project from 1989 through 1994 sponsored by the U.S. Department of Energy, the Gas Research Institute, and the State of Texas, with cofunding and strong cooperation from the natural gas industry. A target of this program was the giant Stratton-Agua Dulce field complex in Nueces County, Texas. It has produced more than 2 Tcf in natural gas from Oligocene Frio reservoirs in a 2,000-ft vertical interval. Static pressure and producing rate from 256 completions were used to determine three reservoir parameters: primary pore volume, barrier transmissibility, and supporting pore volume. Three classes of reservoir compartment size termed large-, medium-, and small-compartment size were evaluated from a spectrum of fluvial-dominated reservoirs classified as bed load, mixed load and suspended load on the basis of facies architecture. These reservoir classes were defined by the frequency distribution of primary compartment size. Recovery factors were evaluated according to different well spacing and completion timing scenarios. Forward stochastic modeling of gas recovery indicated that well spacing of 340, 200, and 60 acres (or less) provided the maximum gas contact efficiency in the three reservoir compartment sizes. The small-compartment-size class (characteristic of suspended-load facies architecture) was predicted to have an incremental recovery potential of 24 percent of OGIP by decreasing the completion spacing from 320 to 160 acres. In the large-compartment-size class (characteristic of bed-load facies architecture), the potential for incremental recovery was indicated to be only 4 percent of the OGIP when the spacing was reduced from 320 to 160 acres per well.
This analysis provides a template for evaluating infield development potential in a quantitative manner. The correlations developed should be transferable to other fields.

Introduction
The Secondary Natural Gas Recovery: Targeted Technology Applications for Infield Reserve Growth of Gulf Coast Reservoirs (SGR) project was a field-based program using an integrated multidisciplinary approach that integrates geology, geophysics, engineering, and petrophysics A major objective of this research project was to develop, test, and verify those technologies and methodologies that have near- to midterm potential for maximizing recovery of gas from conventional reservoirs in known fields. The SGR project aimed at developing and disseminating cost-effective technological applications in each of these disciplines, in order to help maximize the economic recovery of an important infield gas resource. Natural gas reservoirs in the Gulf Coast Basin were targeted as data-rich field-based models for evaluating infield development. Fluvial-deltaic strata of the Frio and Vicksburg Formations in the onshore South Texas Gulf Coast Basin were the site of geologic and engineering investigations of multiple stacked reservoir horizons in a mature gas field (Fig. 1). Evaluating the influence of depositional systems on reservoir heterogeneity and the resulting degree of nonstructurally induced compartmentalization in sandstone gas reservoirs was the goal of the SGR project. Engineering screening of 14 major gas fields in the onshore Gulf Coast Basin indicated that Stratton field was an excellent candidate for detailed investigations within the Frio Fluvial-Deltaic Sandstone along the Vicksburg Fault Zone gas play (FR-4).
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