Development of Shale Gas Reservoirs: Some Subsurface Challenges

Younes, Amgad I.¹
(1)Shell EP Company, Houston, TX.

Shale gas plays recently emerged as a significant reserve replacement alternative in many areas. However, because of their micro-darcy permeabilities, a play’s productivity is limited to presence of connected natural fractures, efficient rock stimulation by hydraulic fractures, or both. The ability to identify the zones which are naturally fractured, or have sufficient stiffness to efficiently stimulate these zones of the resource, enhances the success of a shale play. Meanwhile, the fast-pace development of such resource plays requires a parallel and speedy understanding of key issues that could influence well productivity and eventually play success.

In order to assess some of these issues, we integrate core, image logs, and micro-seismic data in an attempt to sweet spot the Haynesville-Bossier shale play, Louisiana, USA. Our goal was to match production behavior to sweet-spotting criteria. Two elements were assessed: the first is the definition of the depositional environment, reservoir characterization, and facies variations through the analysis of inorganic elements (chemostratigraphy), XRF, XRD, petrography, and the biostratigraphic classification of macro and nano fossil species. The second involved interpretation of the present-day stress and characterization of the fracture system(s) by analyzing 24 cores, 10 image logs, and micro-seismic data. Both parts were then integrated to assist in definition of sweet spots, well planning, and optimization.

The results show that fracture distribution depends not only on the tectonic/basin history, but also on the mechanical layering. Slight variations in rock stiffness, corresponding to changes in clay and silt content, affect fracture distribution in a given layer, and could lead to unusual spacing-length relations. A layered fractured-non fractured shale system is dilemmatic for play development because the highly fractured layers tend to be organic lean, whereas the less fractured layers are organic rich. Depending on the reservoir’s stress profile and horizontal well placement, a layered system can reduce frac efficiency by dissipating frac energy, or enhance it by increasing the stimulated fracture surface area as it connects with reactivated natural fractures.

Utilizing evidence from micro-seismic data, present day stress, mechanical properties and modeling, we discuss the issues above, and suggest models for the development of Shale resource plays.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.