Measurement of Induced Fractures by Downhole TV Camera in Black Warrior Basin Coalbeds

Abstract

Summary Fracture stimulation is commonly used for coal degasification at the Black Warrior basin in Alabama. To understand coalbed fracturing better, Well AM-1 inthe Oak Grove field was completed openhole in the section bracketing the Black Creek coals. Special diagnostics used on this project included variousinjection tests, static-line pressure measurements, and a downhole pressuremeasurements, and a downhole television camera. The television camera observedfractures during injection tests and after the propped fracture treatment. Theauthors believe these are the first successful downhole television pictures ofpropped fractures in coalbeds. propped fractures in coalbeds. Resofts werecompared with predictions of hydraulic fracture simulators. This is a valuableway of "calibrating" hydraulic fracture models for improveddesign/optimization. Introduction In March 1989, special diagnostics were used to gather information on thenature of hydraulic fractures in coalbeds of the Pottsville formation in WellAM-1 in the Black Pottsville formation in Well AM-1 in the Black Warrior basin. Amoco Production Co. and Taurus Exploration Inc. have a joint program toexploit methane gas from coalbeds program to exploit methane gas from coalbedsin the Oak Grove field, from both the Mary Lee/Blue Creek and the Black Creekseams. Amoco's completion has been to set casing through both coal scenes andto place perforations adjacent to each significant perforations adjacent toeach significant seam. This process is followed by two separate hydraulicfracture stimulations with 12/20-mesh proppants-one in the Black Creek series, the other in the Mary Lee/Blue Creek. Although it was possible to make somemodel predictions, the fracture-height growth, the nature of the fracture inthe coal, and the width of the fracture in coal vs.that in bounding zonesremain unknown. Several special well tests and fracturing diagnostics in anopenhole completion were planned to help answer these questions. The planned tohelp answer these questions. The special tests and diagnostic would includemicrofracture stress tests across the openhole interval; various injectiontest, including low-rate injections, step-rate tests (SRT's), minifractures, and a propped fracture stimulation; static-line surface-pressure measurementsduring minifracture and main fracture; and use of a downhole television camerato observe fractures during injection tests and after the propped fracturetreatment. The downhole television camera gave direct measurement of propped fractureheight and width, which served as strong constraints on our fracture modelingefforts. These pictures of propped fractures in coalbeds complement the onlyother direct measurements of propped fractures in coalsthose by mineback. Dataobtained from this experiment include more than 20 hours of videotapes. Oneadvantage of color television is that it makes changes in lithology hourseasier to see. Table 1 summarizes all the testing. Note in particular the twolow-rate injection tests (3 bbl/min), followed by an SRT, two minifractures, and the propped fracture treatment. Downhole Television Camera Downhole television cameras have been used on other occasions to examinefractures. We used a state-of-the-art, color, high-resolution camera* withdownhole and sidescan viewing capabilities (Fig.1). It was 2 1/4 in. in OD, andweights were required to run it in the hole. On some the tests the camerafailed and had to be replaced with a 3 1/4-in.-OD camera with downhole andsidescan viewing but less resolution. When under pressure, both cameras losttheir sidescan rotation, evidently because pressure, affect the rotationalmotor. Although this was a significant loss, it was still possible to view agood number of fractures in the sidescan minor. Scalloping of the televisionimage was a rather serious problem at times during later tests, and theelectronics had to be adjusted continually to improve this. (The contractorsuggested that the problem might have originated in the coaxial cable in thewireline.) Figs. 2A, 2B, 3A, and 3B show various features recorded by the television camera. Measured Fracture Height The production target for the openhole completion interval in Oak Grove WellAM-I is six coal seams in the Black Creek series; the bottom two seams areshown in Fig 4. A density log is shown on the right side, and a gamma ray logis on the left. The hatched boxes in the middle were designated packer settingsfor in-situ stress measurements. For reserves, the most important seam is the Hseam (Point 5). None of the seams are very wide, and the H-seam is the thicker(2 to 3 ft). Net coal thickness is only about 8ft in the entire interval. Fig 5summarizes the sequence of coals and their bounding strata. The openholeinterval was 1,520 ft at the casing shoe to 1,950 ft at total depth (TD). Fracture-height growth increased through the testing sequence (this isdescribed later). The initial camera survey was conducted before any testingbegan (Table 1). P. 270