Completion Techniques for Horizontal Wells in the Pearsall Austin Chalk

Abstract

Summary. Oryx Energy Co. used three basic completion techniques and various combinations of them to complete 20 horizontal wells in the Pearsall Austin Chalk. The completion method selected is based on a Pearsall Austin Chalk. The completion method selected is based on a general set of guidelines. Additionally, equipment selection and various type of workover operations are reviewed. Introduction The Pearsall field is located in south Texas, about 80 miles south-west of San Antonio (Fig.1). Field development of the Austin Chalk begin in the early 1930's. Several drilling "booms" have occurred during the past 60 years. The well count in the Austin Chalk was approaching 2,000 wells by 1982, but at that time, interest in drilling declined sharply. Until 1985, the Pearsall field was developed with vertical characterized by high initial production rates (200+ BOPD), variable PI's (PI=0.5 to 2.0 BPD/psi), steep initial declines (70% to 80% for PI's (PI=0.5 to 2.0 BPD/psi), steep initial declines (70% to 80% for the first year), and low cumulative production (34,000 bbl oil). To continue exploiting the Austin Chalk, all negative vertical-well characteristics had to be eliminated, especially in light of the low oil prices in late 1985, when the additional development of the Austin Chalk began. Horizontal drilling technology was used to address the production problems of a vertical well. Careful consideration for the drilling and completion of horizontal wells has led to the successful redevelopment of the Austin Chalk. Austin Chalk Geology The Austin Chalk formation is a white-to-gray Upper Cretaceous Age limestone with intermixed shale layers. The matrix permeability and porosity are generally low, between 0.01 and 0.25 porosity are generally low, between 0.01 and 0.25 and less than 5%, respectively. The entire Austin Chalk interval is about 500 ft thick in the Pearsall area. The production is associated primarily with small tensional fractures that resulted from downwarping, or subsidence, of the U.S. gulf coast geosyncline during late Cretaceous and early Teriary time,**Available oriented-corer information indicated that the fracture orientation is approximately northeast-southwest. Early Wells. Innovative techniques were required to offset the unpredictable results of vertical wells. Recent work on horizontal well successes influenced efforts to use horizontal drilling techniques in the Austin Chalk. A horizontal well addresses many of the problems associated with vertical wells. A vertical well has to encounter a prolific vertical fracture system to develop commercial reserves. A horizontal well, drilled obliquely to the naturally occurring vertical fractures, has a higher probability of intersecting one of more of these fracture systems. Encountering multiple productive fracture systems with a horizontal well should increase the PI, minimize the problems of the steep initial decline rate, and enhance the estimated ultimate recovery. The primary goals of the first wells were to prove the horizontal wells could be drilled and completed in the Austin Chalk and to show that these wells could have a significant potential for increasing recoverable reserves. The directional technology used for years in offshore drilling operations was extended to drilling the Austin Chalk horizontally. As an alternative, recent developments of slim-hole directional-drilling equipment made re-entry of small-diameter casing possible. The technical issues of each drilling alternative are not discussed in this paper. A newly drilled horizontal well would created the problem of distinguishing between incremental horizontal-well reserves and vertical-well reserves. A depleted vertical well was selected for re-entry because incremental reserves would be attributed to the horizontal well. Selecting the proper candidate for the first horizontal re-entry well was critical. The primary selection criteria were a large casing (5 1/2 in. minimum), high cumulative production, adequate initial stimulation, and a well production rate at an economic limit. Only a small percentage of the Pearsall Austin Chalk wells were completed with 5 1/2 -in. or larger casing. The available directional tools dictated that 5 1/2 in - was the minimum casing size. Average well cumulative production in the Pearsall area was 34,000 bbl of oil. All the vertical wells were fracture stimulated in stages at rates of 20 to 60 bbl/min with 200,000 to 350,000 Ibm of sand on initial completion. Because a well producing at its economic limit has re-covered all its reserves, any additional production would be directly attributed to the horizontal section. Table 1 summarizes the wells available at the onset of the project that had penetrated the Austin Chalk. Based on low cumulative project that had penetrated the Austin Chalk. Based on low cumulative production, Wells B, D, and F were not considered as initial production, Wells B, D, and F were not considered as initial candidates. Well C was still producing in excess of its economic limit and was not considered as an initial candidate because of its remaining reserves. This left two wells for the initial project phase. Well A was selected first because of its cumulative production, which was nearly four times the Pearsall field average. Well A was drilled with short-radius (curve build rate of 2 degs to 3 degs/ft) equipment with a final Austin Chalk exposure of 153 ft. Following a successful stimulation treatment, oil production increased 25-fold. Well E, the second re-entry test, was also drilled with short-radius equipment and had a final reservoir exposure of 270 ft. Oil production increased nine-fold after a stimulation treatment. The production increased nine-fold after a stimulation treatment. The production results of these two wells were encouraging; however, production results of these two wells were encouraging; however, problems encountered during drilling resulted in the actual reservoir problems encountered during drilling resulted in the actual reservoir exposure in either well being less than the 750-ft planned lateral lengths were limited because the wells could not be kept unloaded. The additional fluid head that existed from this condition exceeded the pressure rating of the bottomhole assembly. The availability of medium-radius (curve build rate of 8 degs to 20 deg/100 ft) steerable tools made it possible to drill a longer-reach lateral from an existing well. Well C, which was used for this third lateral, reached the planned exposure of 950 ft and had a production increase of 15-fold without stimulation. This increase could have been higher, but field rules prevented higher producing rates. The success of these three wells was the driving influence for the continuation of the program. The types of completion used for these three wells were varied and were based on the conditions of the well drilled. Completion Types Three basic techniques were used to complete the first 20 horizontal wells, including the three wells discussed above. The three basic methods are openhole (Fig. 2), slotted liner (Fig. 3), and cased hole (Fig. 4). Table 2 summarizes these completions with data on the well type (re-entry or new), completion type, and Austin Chalk exposure. Openhole Completions. Of the first 20 completions, 14 used the openhole method. SPEPE P. 144

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