As the industry seeks to increasingly exploit reserves of natural gas contained in low permeability intercrystalline sandstone and carbonate formations (<20 mD in permeability) many questions have arisen as to the optimum practices to drill and complete horizontal and vertical wells in these systems as well as the best techniques to hydraulic or acid fracture these formations to obtain economic production rates.
This paper provides a summary of recent work which has been conducted in the diagnosis and remediation of problems associated with tight gas reservoirs. Information on the importance of reservoir quality assessment and initial saturation determination is presented as well as a detailed discussion of common damage mechanisms which can affect the productivity of tight gas formations. These include fluid retention problems, adverse rock-fluid and fluid-fluid interactions, counter-current imbibition effects during underbalanced drilling, glazing and mashing, condensate dropout and entrainment from rich gases, fines mobilization and solids precipitation. The impact of these problems during drilling, completion, workover and kill operations is reviewed and suggestions presented for the prevention and potential remediation of these problems.
Specific examples of where these problems have been observed in 23 different common Western Canadian lower permeability gas horizons are presented in a summary format for informative purposes.
Vast reserves of valuable natural gas and associated liquids exist trapped in low permeability intercrystalline and microfractured carbonate and sandstone formations throughout the world. Due to the low inherent viscosity of gas, conditions can be such that these reserves can be recovered from these low permeability strata in situations where the economic recovery of conventional liquid hydrocarbons would be impossible. This paper describes various mechanisms which can influence the effective recovery of gas from low permeability formations and presents a variety of drilling, completion, production and remediation techniques that have proven useful recently in optimizing the recovery of gas from formations of this type.
The definition of a "low" permeability reservoir is somewhat arbitrary, but for the purposes of this paper would be considered to be formations which have a surface routine average air absolute permeability of less than 20 mD. In-situ reservoir condition permeabilities in these types of reservoirs are generally less than 1 mD and can range down into the micro Darcy range (10-6 D) in many situations.
Although the emphasis in this paper is specifically on low permeability gas reservoirs, much of the information presented is also applicable to higher permeability gas bearing formations.
What is the Challenge?
If we consider what could cause uneconomic production rates from a low permeability gas bearing formation, the options generally will fall into six categories, these being;
Poor reservoir quality - period!
Adverse initial saturation conditions
Damage induced during drilling and completion
Damage induced during hydraulic or acid fracturing
Damage induced during kill or workover treatments
Damage induced during production operations