Slator Ranch Fracture Optimization Study
Authors: Ventura J.L. 
Published: December 2007

The Las Ovejas (Lobo) field in Zapata County, TX, is being developed actively. The field was discovered on Tenneco Oil E and P's Slator Ranch lease with the successful completion of the Sanchez-O'Brien Vaquillas Ranch Well 1. Tenneco operates all of the 17,712-acre [71 678 × 10(3)-m2] lease (with the exception of a 320-acre [1295 × 10(3)-m2] tract assigned to the Vaquillas Ranch Well 1) and has successfully completed five wells in the Lobo field subsequent to the discovery well. The Lobo interval in the Slator Ranch area is a tight gas sand, and all these wells require fracture stimulation. Because a successful fracture is essential for a good Lobo completion and because hydraulic fracturing represents a significant portion of the completed well cost, it is important to portion of the completed well cost, it is important to optimize this phase of the completion. The purpose of this study was to determine the following for Slator Ranch Lobo completions:(1)an optimum fracture length as a function of permeability;(2)whether wells should be tailed-in with bauxite, or fractured with all bauxite or sand (if an optimum tail-in does exist, to determine the optimum tail-in for a fixed fracture length as a function of permeability);(3)the drainage area and abandonment pressure permeability);(3)the drainage area and abandonment pressure for Slator Ranch Well(4)the effect of compression on reserves and(5)closure pressure as a function of time and distance along the fracture for Slator Ranch Well 2.

Introduction

Fig. 1 shows the location of Slator Ranch on the Zapata County and Webb County border line. Tenneco has completed five Lobo wells on the ranch since the Sanchez-O'Brien Vaquillas Ranch Well 1, the discovery well for the Las Ovejas (Lobo) field, was completed in July 1981. Fig. 2 shows the location of each of these wells. The Lobo section in which these wells are completed ranges in depth from 10,000 to 11,000 ft [3050 to 3350 mi. The Lobo interval contains three correlative members, the Lobo 1, 3, and 6 sands, all of which have been found to be productive on the Slator Ranch. An average Lobo member contains about 55 ft [17 m] of net sand, each bounded by at least 85 ft [26 m] of shale. The shales are thick, clean intervals and appear to be good barriers to vertical fracture growth. The sands consist of very fine grains of quartz, feldspar, and mica. Quartz overgrowth and dolomite cement the grains. The matrix consists of clays and scattered pyrite. Generally, the mineral composition consists of 30 to 45 % quartz, 5 to 30 % feldspar, 1 to 20 % calcite, and 11 to 25 % clay matrix. Scanning electron microscope (SEM) examination shows that the porosity and permeability of the sands has been porosity and permeability of the sands has been diagenetically altered by quartz overgrowth and clay minerals lining the pore spaces. The permeability of the sands changes drastically over very short intervals. In-situ permeability of the Slator Ranch wells ranges from 0.01 to 0.30 md. In addition to being very heterogeneous, the Lobo sands on Slator Ranch are also intensely faulted. None of the six Lobo completions on the ranch contain three productive Lobo members. Only Slator Ranch Wells 2 and 4 contain two productive Lobo sands; the remaining four wells contain productive Lobo sands; the remaining four wells contain only one productive Lobo sand.

Case History

To research the objectives of this study, as outlined in the summary, the following procedure was set forth.Model and obtain a history match for a Slator Ranch Lobo completion using a reservoir simulator.After a history match is obtained, run a fracture length sensitivity. Assuming that this model reasonably describes other Lobo completions, run additional fracture length sensitivities for various permeabilities.Choose an optimum fracture length for each particular permeability and run a tail-in sensitivity by particular permeability and run a tail-in sensitivity by varying the bauxite/sand ratio in the fracture from 0 to 1 -Choose the optimum fracture length and tail-in for each particular permeability and review the effects of compression.

Slator Ranch Well 2 was chosen as the well to model because it was the only well on the ranch that had both a pre- and postfracture pressure buildup, and because as one of the first wells to be completed, it had more producing history to match. In addition, the permeability of producing history to match. In addition, the permeability of Stator Ranch Well 2 is 0.148 md, which is close to the average permeability of Slator Ranch Lobo wells and therefore typical of an average Lobo completion. The reservoir simulator used for this study was FRACSIM. FRACSIM is a single-phase, two-dimensional reservoir model that can be used to simulate the flow of gas in porous media. Specifically, it was built to analyze vertically fractured gas wells. FRACSIM can be used to investigate various single-well problems such as nondarcy flow, damage around the fracture, wellbore storage, and fracture permeability reduction in time. The only problem with using FRACSIM for this project is its calculation problem with using FRACSIM for this project is its calculation of closure pressure with time.

JPT

P. 1251