Gas Lift Manual

Organic shale reservoirs have a significant part of their hydrocarbon storage system and a poorly quantified part of their transport system in nanometer scale pores in organic matrix material. These pores have a large surface area that strongly adsorbs hydrocarbons. The adsorbed hydrocarbons provide a significant amount of hydrocarbon storage, but also reduce the amount of pore space available for storage of the non-adsorbed hydrocarbons. For methane at reservoir pore pressures, depending on the pore size distribution, the pore space available to store free gas can be more than 30% smaller than the pore space that would be estimated by the standard low pressure helium porosity measurement on ground up samples. The existence of the adsorbed hydrocarbon layer affects transport in two ways. It reduces the space in the organic pores available for free hydrocarbon flow i.e. modifies their permeability, and provides a secondary mode for hydrocarbon transport, diffusion in the adsorbed layer. The permeability alteration is pore pressure dependent and depends on the organic pore size distribution. For methane, calculations show the effect can also be on the order of 30%. Methane storage is investigated by presenting a model that quantifies storage in terms of five parameters, the two Langmuir adsorption parameters, the maximum density of the adsorbed state, the pore volume at zero pore pressure, and pore volume compressibility. The experimental measurement of these parameters is discussed and data from a Barnett core sample is given. For methane transport a transport equation has been developed that is the sum of two coupled terms. The first term is the free gas term that depends on the geometrical permeability and a correction term that accounts for viscosity modification in nano-pores, and wall effects. The second term is a diffusion term that depends on a diffusion coefficient. In the first term adsorption alters both the geometric permeability and the correction term. Both transport mechanisms are driven by the pore pressure gradient.