Horizontal Spacing, Depletion, and Infill Potential in the Austin Chalk (1996)

Abstract There have been more than 4500 laterals drilled in the Austin Chalk. This paper looks at estimated ultimate recoveries (EUR) on a barrels/acre basis for these Austin Chalk wells. Barrels/acre recoveries were computed by estimating ultimate per-well recoveries, drilled density and the impact of vertical production. The data were then analyzed for depletion and infill potential. Certain areas were selected for further study using an artificial neural network. The network was built and used to study the effects of parameters such as lateral length, first production date, structure of the Austin Chalk, etc. on these recoverable barrel/acre numbers. The methodology and regional results of the study are reviewed with detailed analyses shown in selected areas. Introduction The Austin Chalk is a Lower Cretaceous chalk reservoir which extends from Mexico to Mississippi (Fig. 1). Its productive depth ranges from less than 1000 feet to greater than 15000 feet in areas of east Texas and Louisiana. Productive Austin Chalk porosities range from 2% in some of the deeper gas reservoirs to greater than 15% in the shallowest oil reservoirs. Permeabilities range from microdarcies to hundreds of millidarcies. Chalk thickness ranges from less than 50 feet to greater than 800 feet in areas where it produces. This variability leads to confusion as to what constitutes productive rock when doing any type of volumetric analysis. The reservoir heterogeneities are further complicated by an overlying fracture system. This fracture system is typically "well ordered" (Fig. 2). This orderly distribution of fractures causes a large permeability anisotropy (Fig. 3). This anisotropy is a strong reason that the Austin Chalk is a good candidate for horizontal drilling. The fracture system is so laterally extensive in some areas that lost circulation in drilling wells has communicated with producers more than a mile away within a few hours. The lateral extent of these fractures is the upper bound of the area adequately being drained by a horizontal well. History and Scope The first horizontal wells were drilled in the Austin Chalk in the mid 1980's in Pearsall field. Horizontal drilling activity grew rapidly from south Texas into Louisiana. The average recovery from these wells varies greatly from area to area and are as high as 500,000 barrels per well in some areas. Geologic complexities and variability in fracture intensities complicates optimizing well spacing. Actual density is difficult to determine. This causes difficulties in identifying undrained acreage. The methodology described here is a statistical approach used to help identify areas where the Austin Chalk has been overdrilled and areas where infill potential exists. All the techniques have been built in spreadsheets making updating results simpler as new wells are added in an area. The data and results have been used to train and test an artificial neural network in an attempt to use the technique as a predictive tool. Methodology Volumetric reservoir estimates and numerical simulation require a determination of net pay. Because net pay is so difficult to quantify in the Austin Chalk, barrels per acre foot numbers are seldom used. Maps of EUR (estimated ultimate recovery) and barrels per foot of lateral drilled are used to distinguish good and bad areas. These are not ideal for quantitative analysis. P. 409