Fracture Characterization Based on Oriented Horizontal Core From the Spraberry Trend Reservoir: A Case Study
Authors: McDonald Paul , Lorenz John C. , Sizemore Charlie , Schechter David S. , Sheffield Tom 
Published: December 2007

As detailed fracture characterization of the Spraberry Trend continues, there exists an imbalance in the primary sources of fracture data. Measurements of spacing, density, permeability and orientation have traditionally been calculated or inferred from pulse and/or tracer type tests. To date, no direct measurement of Spraberry fracture properties from core in a horizontal plane are known. Approximately 400' of horizontal core was recently taken from the two main pay sections in the Upper Spraberry (1U and 5U) in Midland County, Texas. This paper will describe the acquisition, orientation, and subsequent analysis of the first ever horizontal core taken from the Spraberry Trend.

Introduction

Natural fractures existing over a regional area have long been known to dominate all aspects of performance in the Spraberry Trend Area. However, there is little or no information on the actual fracture system other than: orientation, on a gross basis, from pulse and/or tracer tests in the 50's and 60's, and fracture spacing inferred from simulation and a few existing vertical cores. Previous descriptions and old core reports did not distinguish between natural and coring induced fractures, thus almost all information from the early years, when almost all Spraberry data was obtained, provides no detailed information on the natural fracture system. The first vertical core, taken as part of the current program in 1993 from the Shackelford Spraberry Unit #1–38A, intersected a vertical natural fracture with significant mineralization that had clearly grown into unoccupied space. This open, mineralized fracture was contained within a thin pay sand and was observed to terminate at a shale parting. This fracture was the first documented evidence of the existence of a mineralized natural fracture within the pay sand.

The orientation, containment within zone, degree of mineralization, fracture aperture and spacing are important questions when considering fluid flow in naturally fractured reservoirs. However, after considerable data gathering, it became apparent that only superficial characterization of the natural fracture system was available. Recent acquisition of the horizontal core has radically altered understanding of the natural fracture system in the Spraberry Trend Area. This well, the E.T. O'Daniel #28, was cored with the intent of intersecting natural fractures in the thin sand streaks where oil saturation is found in the Upper Spraberry. Over 100 natural fractures were intersected exhibiting an intriguing and diverse array of fracturing behavior.

Geology

The Spraberry formation was deposited during the Permian age within the Leonardian system in the Midland Basin, a part of the province known as the Permian Basin of West Texas and Southeast New Mexico. The depositional environment consisted of a combination of turbidity and density currents sourced from the northwestern shelf area. The Spraberry formation facies can be characterized as submarine fan and basin plane deposits and consists of very fine grained sandstones to coarse grained siltstones and mudstones. The various fan complexes can be traced to the north in Dawson and Lynn counties and are considered the source material for the fan lobe sequences within the area of study in eastern Midland County.

Since its discovery, the Spraberry has been subdivided into three principal intervals, the lower, middle and upper Spraberry formations. In the late 1980's, Tyler and Gholston further subdivided the Spraberry intervals to represent distinct episodes, or operational units, within each of the submarine fan complexes (Figure 1). Six of these operational units were found in the Upper Spraberry and two in the Lower Spraberry. Only two of the six operational units in the Upper Spraberry (1U and 5U) have been identified as containing reservoir quality rock capable of making significant production contributions.

The proposed target of the horizontal core was the natural fractures within the main pay intervals of the 1U and 5U sands. Net pay within each of these zones averages between 8 and 14 ft.

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