Abstract

Fractured reservoirs that produce oil by solution gas drive have become popular candidates for development with horizontal wells. Of the 2,000 horizontal wells drilled across the U.S. through the end of 1991, nearly 1,900 targeted vertically fractured reservoirs. Previous to horizontal development, most reservoirs were developed with various densities of vertical wells. Horizontal wells are often drilled near better vertical wells and some relationships between the two well types have been noted. However, horizontal drilling results have been quite variable and unpredictable.

Substantial production data from vertical and horizontal wells in the Austin Chalk (Pearsall Field, Texas), Niobrara Chalk (Silo Field, Wyoming), and the Bakken Shale in western North Dakota are now available. These data allow statistical descriptions of fracture systems and prediction of production performance for initial rate and expected reserves as a direct function of statistically derived, quantitative measures of reservoir heterogeneity and depletion at the time of horizontal drilling.

Fractured reservoir heterogeneity can be determined from vertical well oil recovery. In this paper, heterogeneity is defined by 1.) the Dykstra-Parsons variation coefficient (V), based on the distribution of vertical well oil recoveries in a given play and 2.) the ratio of the maximum vertical well recovery to the median vertical well recovery. As fracture heterogeneity increases from homogeneous to very heterogeneous, V increases from 0.55 to 0.90 and the maximum/median ratio increases from 3/1 to 25/1. Horizontal to vertical ratios or “multiples” of initial production rate and ultimate oil recovery are directly related to these measures of heterogeneity as shown by:

1.) Relatively homogeneous reservoir fracture systems such as those of the Bakken Shale in North Dakota (V = 0.55, maximum/median = 3.3/1) exhibit a horizontal/vertical initial production rate ratio of 2.6/1 and a reserve ratio of 1.1/1 for low prior vertical depletion of less than 100 BO/acre.

2.) The Niobrara Chalk in Silo Field has intermediate heterogeneity over its productive fairway (V = 0.80, maximum/median = 7.4/1) and has a horizontal/vertical initial production rate ratio of 3.0/1 and a reserve ratio of 1.7/1 for moderate prior vertical depletion of 118 BO/acre.

3.) Highly heterogeneous reservoirs such as the Austin Chalk at Pearsall Field (V = 0.90, maximum/median = 25.6/1) have horizontal/vertical initial production rate ratios of 4.6/1 or more depending on depletion and reserve ratios of 3.1/1 for high vertical depletion to 6.0/1 for moderate vertical depletion of 87 BO/acre.