Evaluation of Devonian Shale With New Core and Log Analysis Methods
Authors: Luffel D.L. , Guidry F.K. , Curtis J.B. 
Published: December 2007

Results of this study of Appalachian basin Devonian shale show that allporosity exceeding 2.5% is occupied by free hydrocarbon (mostly gas). Fromanalyses of logs and 519 ft of conventional core in four wells, reservoirporosity averages 5% and free-gas content averages 2% by bulk volume.

Introduction

There is a growing interest in shale formations in selected areas of the U.S. as potential commercial gas and oil reservoirs. An example is the Devonianshale in the Appalachian basin, which is considered a major potential gassource. The Gas Research Inst. (GRI) sponsored a research program to improvethe understanding of formation evaluation and reservoir description in shalesand to enhance well productivity and gas recovery through better wellcompletions and stimulation methods. Four air-drilled wells were completedrecently in the Devonian shale. Extensive formation evaluation data, including519 ft of conventional cores, were collected from these wells. Designatedcomprehensive study wells (CSW's), the four wells are located in Kentucky and West Virginia.

One purpose of the CSW program was to develop new formation evaluationmethods to identify gas-producing intervals both for deliverability andreserves with logging tools. Significant progress has been made. A key elementof these new evaluation progress has been made. A key element of these newevaluation methods is to establish the reservoir rock and fluid propertiesthrough careful core analyses and then to develop the log interpretationmethods necessary to match core and production results. New core analysismethods had to be developed to provide accurate measurements of reservoirporosity, gas, oil, provide accurate measurements of reservoir porosity, gas, oil, and water content. These new methods involve crushing the rock samplesbefore extraction, drying, and measuring porosity. In the black, organic-richshales present in these wells, porosities measured with the new methods werehigher by more porosities measured with the new methods were higher by morethan a factor of 3 and free-gas contents were higher by up to a factor of 20compared with conventional methods.

Through use of these new core and log analysis methods, an extensivedatabase was developed for reservoir description in the four CSW wells. Severalimportant features have emerged from interpretation of these data. Results arepresented from a relatively new plotting method that relates bulk volume ofhydrocarbon to porosity. This valuable tool has provided useful insight intothe fluid distribution present in the reservoir and clearly indicated theminimum porosity required to store free hydrocarbons. Also, this plottingmethod leads to a means of determining formation resistivity factor as relatedto porosity and of controlling quality to monitor core and log analysisresults.

In the Devonian shale, an important part of formation evaluation by loganalysis is to determine the amount of kerogen present because kerogen appearsas hydrocarbon-filled porosity on present because kerogen appears ashydrocarbon-filled porosity on conventional logs. In this study, total organiccarbon (TOC) and pyrolysis analyses were made on 93 core samples from the four CSW pyrolysis analyses were made on 93 core samples from the four CSW wells. Anew method was used to derive volumetric kerogen and oil content. Results(shown here) then were used to derive kerogen from the uranium response of thespectral gamma ray log. In addition, free-oil content is shown to relate tokerogen content. This is useful in partitioning the reservoir free gas and oilbecause they are indistinguishable from logs in the Devonian shale.

Finally, results of salinity measurements made directly on 50 core samplesfrom the CSW wells are shown to determine formation water salinity for loganalysis. This information is very important because formation water rarely isproduced from the Devonian shale.

Porosity and Fluid Content Porosity and Fluid Content In the new method ofcore analysis, a whole core piece (about 300 g) is weighed, bulk volume ismeasured by immersion in mercury, and bulk density with contained fluids iscalculated. The core piece is then crushed and blended, and a measured weightof about 100 g is taken with a sample splitter. The crushed sample is extractedwith toluene (Dean Stark) for 1 to 2 weeks and dried at 230F for 1 to 2 weeksuntil stable. The grain volume is then measured with helium. Crushed-samplebulk density is presumed to be the same as that measured on the whole corepiece.