Infill Drilling & Reserve Growth Determination in Lenticular Tight Gas Sands
Authors: Cipolla Craig L. , Mayerhofer Michael 
Published: December 2007

Abstract
The paper details the acquisition of detailed core and pressure data and the subsequent reservoir modeling in the Ozona Gas Field, Crockett County, Texas. The Canyon formation is the focus of the study and consists of complex turbidite sands characterized by numerous lenticular gas bearing members. The sands cannot be characterized using indirect measurements (logs) and no reliable porosity-permeability relationship could be developed. The reservoir simulation results illustrate the problems associated with interpreting typical pressure and production data in tight gas sands and details procedures to identify incremental reserves. Reservoir layering was represented by five model layers and layer permeabilities were estimated based on statistical distributions from core measurements.

Introduction
The ability to accurately evaluate infill drilling in tight gas sands is many times limited by the complex nature of the reservoirs and the scarcity of data. The wells are usually fracture stimulated, which precludes gathering accurate permeability and pressure data after stimulation. The sands are many times discontinuous or compartmentalized, consisting of numerous sand-silt-mud sequences from inches to several feet thick. In many tight gas fields the complex depositional and diagenetic history of the sands does not allow simple porosity-permeability correlations, resulting in an inability to predict well productivity from logs. The general absence of reliable pressure and permeability data requires that infill drilling studies rely on volumetric estimates of original gas in place (OGIP) and permeability estimates from well performance.
This paper will examine reservoir layering and its effect on reserve growth determination using a 5-layer multi well reservoir model. Reservoir layering has been studied by Fetkovitch in more permeable, continuous reservoirs in the Hugoton gas field, concluding that minimal incremental reserves were attributable to infill drilling. The focus of this work is geologically complex reservoirs with layer permeabilities of 0.2 mD or less. Individual layers cannot be distinguished or mapped from well-to-well, requiring that reservoir layering be described using pseudo layers with permeabilities based on statistical data from core measurements. In most cases sufficient permeability data is not available to conduct reliable geostatistical studies of vertical and areal heterogeneities.
To improve reserve growth estimates, layer pressures and permeability distributions are required to support the reservoir modeling. These data are normally only available on a limited basis and therefore must be extrapolated to the entire field. A case history from the Ozona Canyon sands is used to illustrate the integration of layer pressures and core data with multi well reservoir modeling to estimate incremental reserves with 40 acre infill drilling.

Ozona Canyon Sands
The Ozona Field is located in Crockett County, Texas (reference Figure 1). The Canyon sands are complex turbidite deposits with permeabilities from 0.0001 mD to 0.2 mD at depths of 6,000-7,500 ft. There are more than 1500 Canyon completions in the Ozona Field, covering about 150,000 acres. Wells are typically fracture stimulated using 50,000 to 250,000 lbs of sand and will produce 50 to 5,000 MMCF of gas. The field was initially developed on 320 acre spacing in the 1960's, with subsequent development on 160 and 80 acre spacing. This study was conducted to estimate reserve growth with 40 acre development that was subsequently approved in 1995. The work was supported by detailed petrophysical, fracture technology, single well simulation, and full-field production studies.
A field-wide study identified areas of the Ozona field with small drainage areas per well, yet showing both declining initial pressures and well recoveries as infill development progressed (1960's to 1990's). Reservoir layering was suspected, with higher permeability layers drained by initial development wells, while large reserves remained unrecovered in the less permeable layers. Layered behavior was confirmed by special layer pressure measurements in new wells prior to stimulation. P. 533