Pearsall Field Research Articles

Geologic characterization of the type cored section for the Upper Cretaceous Austin Chalk Group in southern Texas: A combination fractured and unconventional reservoir

A rock-based geologic characterization was completed on a continuous core through the Austin Chalk Group section, an active exploration target in southern Texas. Because this core (located in the Pearsall field) is continuous and includes both lower and upper contacts, it can be used as the type cored section for the southern Texas Austin Chalk. Several general lithofacies are defined. Two lithofacies (lithofacies 1 and 2) are highly bioturbated and have low to moderate organic matter content and are poor to moderate source rocks. Two other lithofacies (lithofacies 3 and 4) are laminated, lithofacies 3 containing some burrows and lithofacies 4 containing no burrows. Both lithofacies are good source rocks. Organic matter in the laminated lithofacies 3 and 4 and some of the organic matter in the bioturbated lithofacies 2 are dominated by type II kerogen, suggesting the section can be self-sourcing. A general trend is observed whereby lithofacies that were deposited under more-oxygenated conditions increase in abundance up section and is related to a decrease in total organic carbon and, hence, source-rock quality. The primary oil and gas production is from fractures with possible later production from the nano- to microporous matrix after pressure drawdown. Pore types are predominantly interparticle nano- to micropores and lesser intraparticle nanopores as defined by Loucks et al. (2012). Porosity is less than 8%, and permeability is less than 0.05 md and ranges down into the nanodarcy range. The type cored section adds to the understanding of the southern Texas Austin Chalk by providing a geological description of lithofacies, stacking patterns, source-rock quality characterization, and reservoir quality. The type core can be used as a comparison section to other cored sections of the Austin Chalk.

Characterization of Opening Mode Fracture Systems in the Austin Chalk

ABSTRACT The Upper Cretaceous Austin Chalk is a low-permeability, fractured reservoir that has been the target of numerous horizontal wells in Texas. For wells to be successful, parameters such as well-bore azimuth, target horizon, and length must be optimized. Furthermore, information on fracture height and the storage volume potentially connected to the well bore is required. Two case studies are presented in which the height, aspect ratio, and vertical persistence of opening-mode fractures in the Austin Chalk have been examined, together with previously known attributes of fracture population attributes of orientation, aperture, effective aperture, spacing, aspect ratio, fracture fill, and fracture permeability. The studies are of an outcrop of the upper Austin Chalk near Waxahachie, North Central Texas (Grove Creek), and of a vertical segment and two horizontal laterals of a core in the Atco Member in Frio County, Pearsall Field (Kinlaw core). Large, potentially open fractures are commonly clustered, the distance between clusters ranging from ~1 to ~50 m. Aperture-size distributions follow power laws, and spacing-size distributions are negative-logarithmic or lognormal. The aperture size at which fractures are open to fluids is variable (0.14 to 11 mm). Fracture permeability, which is scale dependent, has been determined at 7.1 D (for 18 m of lower Austin Chalk core) and 286 D (for 300 m of upper Austin Chalk outcrop). Fractures may terminate at chalk-marl contacts, tip out within the marl, or pass through the marl layers. Fracture height is governed by truncation effects at marl horizons and pressure-solution seams, which in turn partly depend on fracture aperture and thickness of the inhibiting layer.

Horizontal Slim-Hole Drilling With Coiled Tubing: An Operator's Experience

Summary What is believed to be the first horizontal well drilled with directionally controlled coiled tubing recently was completed in the Austin Chalk formation. An existing well was sidetracked out of 4 ½ -in. casing with a conventional whip stock. An average build rate of 15°/ 100 ft was achieved in the curve, and a 1,458-ft vertical section was drilled with 2-in. coiled tubing, downhole mud motors, wireline steering tools, a mechanical downhole orienting tool, and 3 7/8-in. bits. This paper discusses the orienting and directional tools and techniques developed during this operation. It also describes improvements made for the second well. Introduction The Austin Chalk formation within the Pearsall field in Texas consists of a Cretaceous limestone through which production occurs from natural fractures. Drilling through these fractures requires careful preparation because the rig may encounter prolific flow of oil or complete lost circulation. During the discovery of the Pearsall field, wells were drilled vertically to the Austin Chalk. Horizontal drilling currently is the preferred technique for development of the Austin Chalk. Through horizontal drilling, a single wellbore can intersect several fractures, potentially increasing initial flow and ultimate oil recovery. Fresh water usually is used as a drilling fluid because it will control formation pressures when circulation is lost. Surface pressure containment equipment allows the flow of well fluids and gases to the surface under controlled circumstances where they are processed during drilling. In the field, this technique is commonly called "flow drilling." This method minimizes loss of drilling fluid to the formation and increases rate of penetration (ROP). The severity of either lost circulation or prolific flow can pose operational and safety problems. Loss of circulation can prevent adequate removal of drill cuttings, increasing the potential for sticking the drillstring. In addition, the logistics for providing an ample water supply in this semiarid area are complex because the large volumes required to power the downhole mud motors are lost to the natural fractures. Prolific flow from the fractures necessitates the handling of large volumes of oil and gas at pressures that can approach the limits of the surface equipment. Safely tripping into or out of the hole with drillpipe requires killing the well, which sometimes calls for the use of expensive and environmentally undesirable heavy brines. Therefore, a method of tripping while under pressure without killing the well would greatly enhance flow drilling operations. This problem was addressed by replacing the conventional drillstring with coiled tubing.

Development of the Austin Chalk in Giddings Field, Burleson County, Texas, by Horizontal Drilling

ABSTRACT The Austin Chalk produces in a NE-SW trend, sub-parallel to the Texas Gulf Coast, in a band approximately 523 km. (325 mi.) long and 40-64 km. (25-40 mi.) wide. Giddings Field is located in the central part of this trend and covers a portion of several counties along the productive fairway. The Austin Chalk produces from a hard, dense, intensely fractured limestone with a matrix porosity range of 3-8% and permeabilities of less than 0.01 md. Production comes primarily from the fractures with contribution from the matrix coming later in the life of the well. The Austin Chalk was deposited near the southern end of the Cretaceous western interior seaway during the Coaniacian and Santonian ages (late Commachean to early Gulfian series) on a stable, gently dipping ramp during a high stand of sea level. The Austin was deposited as a calcareous ooze made up of coccoliths, coccolith fragments, planktonic foraminifera, and fine grained skeletal debris. The Austin is poorly to intensely burrowed and contains organic rich interbedded shale layers with gradational to sharp contacts with the intervening cleaner limestones. The high total organic content indicates the potential for the Austin Chalk to have been partially or completely self-sourced. As a result of the development and improvement of horizontal drilling techniques, the Austin Chalk has experienced a flurry in drilling activity. This boom began in Pearsall Field in the southern end of the trend and soon spread to the northeast to Giddings Field and beyond. Several different methods such as drilling a highly deviated hole or traverse across the Austin, drilling parallel to faulting, stair-stepping across different zones of interest, or drilling in a specific target for the entire well path have been tried (along with other ideas) with varying degrees of success. There are many different questions to be answered before drilling a horizontal well. Consideration should be given to off-set production, presence or absence of faulting, and thickness of the target zone to name just a few. The parameters and guidelines used in one area may not work the same way in a different area or in a different part of the same field. Seismic should be used in choosing locations and in evaluating areas of interest where there is limited subsurface control. For a typical horizontal chalk well, casing is set into the top of the Austin 6-9 meters (20-30 feet) at some predetermined angle which will allow for getting into the target zone as soon as possible. If needed, a tangent section is then drilled before continuing to build angle to reach the center of the target. The well is then drilled to the planned measured depth and completed. Early wells were cased but due to cost and the nature of the reservoir the subsequent wells were completed open hole. The early horizontal wells were drilled with lateral sections less than 609 meters (2000 feet). Improvements in drilling techniques have enabled operator to drill lateral sections greater than one mile. Both old and new methods are used for formation evaluation of the horizontal wells. Careful evaluation of the cuttings and close monitoring of the penetration rates and gases are critical in determining if the lateral section is being drilled in the right zone in identifying fractured intervals and in qualifying shows. New borehole imaging tools produce a visual record of the borehole and are good direct measurement indicators of fracturing along the well path. The data gathered from these logs can be used in many ways to aid in evaluation of the play. End_of_Record - Last_Page 801-------

Drilling and Production Aspects of Horizontal Wells in the Austin Chalk

Summary. This paper discusses testing of horizontal technology for use in the highly fractured Giddings field. Three short- and seven medium-radius wells were drilled successfully in the Austin Chalk formation. The paper discusses well plans, bottomhole assemblies, trajectory control, telemetry, mud systems, hydraulics, hole cleaning, casing design, cementing, problems encountered, formation evaluation, completions, and reservoir response. Introduction The Austin Chalk formation of the Giddings field in central Texas (Fig. 1) is an Upper Cretaceous fractured limestone oil and gas reservoir that was developed in the late 1970's and early 1980's. Initial development was accelerated by the explosive oil prices of the time and by the earlier development of the Pearsall field. Although the Giddings field was discovered in 1961, its potential was not evident until 1973, when re-entry into the Giddings No. 1 flowed 300 BOPD. Field development began in 1976 and accelerated rapidly. Because of the field's highly variable natural fracture system, initial performance of vertical wells ranged from highly prolific to exceptionally poor. Field development declined rapidly after 1982, partly because of failing oil prices. By 1984, reservoir studies and stimulation analyses had revealed why recovery efficiency from existing vertical wells remained low: the vertical wellbores were linked to only one natural fracture system, leaving unpenetrated fracture systems and matrices poorly drained. Instead of drilling infill vertical wells to drain the reservoir, we tested horizontal drilling and completion technology to determine whether multiple natural fracture systems could be linked to a single wellbore. Since 1985, a total horizontal displacement of 14,500 ft has been drilled within the pay. The longest horizontal displacement, pay. The longest horizontal displacement, 2,200 ft, was drilled within a 30-ft target. Geology and Background The Austin Chalk produces along a trend 10 to 25 miles wide and 250 miles long from Mexico to Brazos County, TX, with scattered production in east Texas. The most prolific area to date is the Giddings field, covering 1,200 sq miles in Burleson, Lee, Fayette, and Washington counties. Structural strike in the Giddings field is northeast, with the productive portion of the field ranging from 6,500 to portion of the field ranging from 6,500 to 12,000 ft. Porosity ranges from 3 to 12%, with very low matrix permeability (less than 0.1 md). Net pay thickness ranges from 10 to 240 ft. Reservoir development in the Austin Chalk is almost wholly the result of fracturing, creating a permeability system of near-vertical fractures that run parallel to structural strike (i.e., northeast/southwest). Because these linear fracture systems, which connect the tight matrix with a wellbore, are isolated flow channels, only limited areas can be drained by a vertical wellbore. Horizontal drilling, however, connects multiple vertical fracture systems with a single perpendicular wellbore to drain a larger area perpendicular wellbore to drain a larger area at higher producing rates. All 10 horizontal wells targeted the bottom portion of the Austin Chalk because of its abundant fractures, greater oil saturation, and proximity to the Eagleford, which also may be a major source-rock contributor of hydrocarbons. Definitions for Horizontal Wells The popularity of horizontal drilling during the last few years has led to use of the term "horizontal drilling" for many conventional directional-drilling operations, river crossings, extended-reach drilling, and other directional applications. We think horizontal drilling should be defined as a drilling and completion technique in which the wellbore remains in a high-angle trajectory roughly parallel to the formation, thereby exposing significantly more pay to production than would be exposed by a vertical production than would be exposed by a vertical wellbore. Many of the more recently developed, aggressive curve-building techniques are not actually horizontal drilling unless the lateral wellbore remains in the target for a distance 10 or more times the thickness of the net pay. In many reservoirs, a lateral length 20 to 50 times the thickness is necessary to make horizontal drilling economically viable. JPT P. 773

Opportunities for Horizontal Drilling in Texas

To date, horizontal drilling in Texas has been effective in achieving production increases where natural fracturing is significant, as in the Pearsall field of the Austin Chalk trend of south Texas. Projected average ultimate recoveries of 500,000 bbl of oil and 500 mmcf of gas per horizontal well compare with 75,000 bbl and 82 mmcf for vertical wells in that field. In addition to increasing production from naturally fractured reservoirs, horizontal drilling applications can enhance production where coning problems exist, where the reservoir zone is thin, where a gravity drainage mechanism exists, or where macroscale heterogeneity can be overcome to produce from complex, compartmentalized reservoirs. Major Texas reservoirs have been classified into 47 oil plays and 73 gas plays that are useful in evaluating the application of horizontal drilling. Geological character amenable to horizontal drilling is evident in at least 10 oil plays containing 144 reservoirs, more than two Bbl of proved reserves, and eight Bbbl of uncovered mobile oil. At least seven gas plays containing more than 124 reservoirs with cumulative production of more than 19.7 tcf are amenable to horizontal drilling. Tight gas reservoirs may benefit from multiple hydraulic fracture treatments from horizontal well sections drilled parallel tomore » the minimum horizontal stress direction. Although play analysis defines general suitability, applications of horizontal drilling depend upon geologic, engineering, and, most importantly, economic assessments of specific prospects. Increased understanding of fractured reservoirs and of scales of reservoir heterogeneity will enhance future applications.« less

Horizontal Exploitation of the Upper Cretaceous Austin Chalk of South Texas

Horizontal drilling in the fractured Austin Chalk of south Texas has proven to be a viable technology for exploiting reserve opportunities in mature trends as well as in frontier areas. To date, the results of an interdisciplinary approach to the regional analysis of structure and stress regimes combined with studies of the depositional characteristics of the Austin Chalk and Eagleford Shale have been a success. Productive characteristics of the Austin Chalk indicate the influence of regional fractures on the preferential flow direction and partitioning in the Pearsall field area of the trend. Well bore orientation and inclination are designed such that multiple fracture swarms at several stratigraphic horizons are intersected with a single horizontal well bore. As a result of the greater frequency of fracture contacts with the well bore, there is a significant increase in the ultimate recovery of hydrocarbons in place. Conventional vertical drilling techniques are frequently ineffective at encountering these laterally partitioned fracture sets, resulting in lower volumes of recoverable hydrocarbons. Additionally, horizontal well bores may increase ultimate recovery of hydrocarbons by lowering the pressure gradient to the well bore and maximizing the reservoir energy.

Austin Chalk Fracturing Design Using a Crosslinked Natural Polymer as a Diverting Agent

This paper discusses a successful fracturing technique used to stimulate the Austin Chalk formation, located in Dimmit and Lee counties in south Texas. Austin Chalk geology and properties of reservoir fluids are described, and other fracturing practices are discussed. Production results from various stimulation techniques are compared. Introduction During the past 2 years, considerable interest has been shown in producing the Austin Chalk formation in south Texas because of higher oil paces and increased demand. The Austin Chalk formation in the Pearsall Field of Frio County, TX, was discovered in 1933 and partially developed through 1941. Activity briefly resumed between 1948 and 1956. Recent activity began in late 1974 and continues today. Austin Chalk operations cover an area from Dimmit County to the west through Robertson County to the east (Fig. 1). Within this area, Pearsall Field (Frio County) and West Dilley Field (located in parts of Zavala, Dimmit, LaSalle, and Frio counties) are the most active locations. Operations also have begun in Lee and Burleson counties. Stimulation practices during the first two operating periods primarily consisted of detonating nitroglycerin periods primarily consisted of detonating nitroglycerin through the heavily fractured intervals, followed by 2,000 to 5,000 gal HCl acid. Although a limited amount of hydraulic fracturing began in 1955, activity in the Pearsall Field terminated shortly after because of Pearsall Field terminated shortly after because of unsatisfactory economics. However, recent interest in the Austin Chalk has caused a demand for stimulation technology to make production from this formation economically feasible. This paper describes a technique of hydraulically fracturing the Austin Chalk formation to increase productivity by assuring that the complete vertical formation productivity by assuring that the complete vertical formation interval is fractured. By improving productivity using this fracturing technique, the economics of producing the Austin Chalk formation also are improved greatly. Theory To obtain maximum production from thick, low-productivity formations such as the Austin Chalk, the entire interval must be fractured. This exposes more fracture surface area to the drainage system than when fracturing only limited intervals of the thick producing section. High pump rates and large fluid volumes alone cannot fracture pump rates and large fluid volumes alone cannot fracture the entire vertical length of this formation. However, fracture diversion techniques combined with high rates and volumes can do this and can be verified by temperature logs. The technique of using a crosslinked natural polymer as a diverting agent is an attempt at both casing and formation diversions. High-rate, high-volume stages of fracturing fluid are pumped for maximum penetration, followed by volumes of a crosslinked natural polymer having the properties of a Bingham plastic. This gel allows increased hydraulic pressure to be applied to adjacent intervals, thus diverting the fracture propagation so that extensive vertical intervals may be fractured. Ball sealers also are used after one stage of the procedure to assure further casing diversion than that achieved with gel only. Using crosslinked natural polymers for diversion in massive, highly fractured formations is not unique to the Austin Chalk formation. JPT P. 1795

The Stratigraphy and Structure of the Pearsall Field, Frio County, Texas: ABSTRACT

A history of the development of the Pearsall field is given. The stratigraphy of the Cretaceous section encountered by the wells at Pearsall is discussed and the correlation is made with the standard fault-line section. Structure contours are drawn on top of the Austin chalk. End_of_Article - Last_Page 1514------------