The Jackfork sandstone formation is widely distributed in the Ouachita Mountains in southeastern Oklahoma and southwestern Arkansas, USA. It has become of interest in recent years because substantial gas reserves have been found. As the formation is very tight, hydraulic fracturing stimulations are normally required. A systematic investigation of the petrophysical and geomechanical properties of the formation is needed to assure the success of future exploration and development of these reserves. This paper presents laboratory measurements of such properties. Petrophysical properties measured include porosity, permeability, bulk density, grain density, and seismic velocity; some of them were measured in both horizontal and vertical directions. Geomechanical properties determined are uniaxial tensile strength, uniaxial compressive strength, triaxial compressive strengths under three different confining pressures, Young's modulus, Poisson's ratio, cohesion, the angle of friction, and Mode-I fracture toughness. A brief comment is included for each property measured on its potential applications in future reservoir exploration and development. In addition, a detailed introduction is given on the upgrading and application of the CDISK method for Mode-I fracture toughness measurement on small samples.
1. INTRODUCTION
The Jackfork sandstone formation has become of interest in recent years because substantial gas reserves have been found in it [1]. As the formation is very tight, hydraulic fracturing stimulations are normally required. Among the 15 wells initially drilled, 13 were hydraulically fractured and completed successfully; but the other two failed, partly due to the lack of proper information on the petrophysical and geomechanical properties of this formation. The work presented in this paper was designed to fulfill that task, together with the introduction of an upgraded method for measuring fracture toughness on small samples.
2. GEOLOGY OF JACKFORK FORMATION
The Jackfork sandstone was named for the Jackfork Mountain in the Ouachita Mountains located in Pittsburg and Pushmataha counties, Oklahoma, USA. It is a Pennsylvania formation that extends from southeastern and central southern Oklahoma to southwestern Arkansas in the Ouachita Mountains area (Fig. 1) [2].
(available in full paper)
While current gas exploration activities are all within Oklahoma, it is believed that gas reserves will also be discovered on the Arkansas side [1]. For this reason and due to the accessibility to the outcrops, samples for this work were picked from the R.D. Plant Quarry in Kirby, Arkansas.
3. PREVIOUS PETROLEUM ACTIVITIES
The first discovery well in the Jackfork sandstone was drilled in 1992 in southern Latimer County, Oklahoma. Gas was first hit in the lower Jackfork sandstone at the depth of 3,612 m. The second discovery well was drilled about 5 km away. It encountered several productive intervals in the upper Jackfork sandstones. A third well was drilled 16 km away from the first one. This well confirmed the discovery of gas reserves. Sidewall sample analyses found the permeability is extremely low; and hydraulic fracturing stimulation is required [1].
4. PETROPHYSICAL PROPERTIES
4.1. Porosity, Bulk Density and Grain Density The bulk volume of rock consists of two parts: the grain volume and the pore volume. Porosity, ø, is the percentage of pore volume within the bulk volume.
