Abstract

The anomalous ability to produce natural gas from very low permeability (<0.05 mD in-situ) intercrystalline matrix sandstones and carbonates has been welldocumented on a worldwide basis. This paper illustrates that, in general, for a condition of economic gas production to occur, the initial water saturation that exists in the productive pay must be significantly lower than what would be expected from a normal capillary pressure equilibrium if the matrix under consideration were in dynamic contact with free water. A number of mechanisms for the establishment of these unique noncapillary equilibrium saturation conditions have been postulated over time. This paper presents the theory of long-term regional non-equilibrium gas migration and desiccation as an explanation for this phenomenon, and provides both field and laboratory data that corroborate this mechanism of subnormal initial saturation condition establishment.

Introduction

With declining production and reserves of 'conventional' natural gas production in higher permeability/pressure formations (1 mD plus), considerable interest and success has been centered in recent years on the exploitation of vast reserves of natural gas contained in 'tight' and 'ultra-tight' sandstones and carbonates. These formations have 'surface' clean core air permeability values generally ranging in the 0.005 to 0.5 mD range and effective 'in-situ' reservoir condition permeability to gas values often in the 0.0001 to 0.05 mD range1.

The ability of a very low permeability matrix to produce reservoir fluids is controlled by the interaction of several parameters when considering the economic viability of gas production. They include:

  1. The absolute permeability of the porous media, dictated by the combination of pore system geometry and interconnectiveness, pore throat size distribution and presence of clays and other authigenic or detrital infilling materials.

  2. The degree of overburden pressure/compression effect on the pore system geometry. For low permeability pore systems, this effect is often substantial (1–2 orders of magnitude or more reduction between unstressed 'routine' type surface gas permeability measurements and fully in-situ stressed measurements).

  3. The presence/absence of natural micro, meso or macro fractures and the condition (open or infilled) and orientation/interconnectiveness of this natural fracture system.

  4. Relative permeability effects associated with the presence of initial trapped or mobile immiscible fluid saturations (oil, water, bitumen, pyrobitumen) present concurrently with the gas in the pore system.

  5. Wettability of the porous media controlling the specific distribution of the initial immiscible phase saturations within the low permeability intercrystalline matrix.

For tight gas systems, Criteria 1 and 2 are generally such that the resulting in-situ permeability falls into the 0.001 to 0.05 mD range. In many tight gas systems, a natural fracture network exists, which is usually required in order to access sufficient matrix drainage area to allow economic production rates. In non-naturally fractured systems, large hydraulic fracture treatments (generally 100 or more tons of proppant placed) are usually required in order to access sufficient drainage area.

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