Depleted zones are intrinsic to most mature reservoirs throughout the world. The associated issues of effective, safe and economically viable well construction, completion and workover within low pressure environments become more challenging on a daily basis.
This paper discusses the application of aphron drilling fluid technology, which offers a unique alternative option that can significantly improve the operational and economic aspects for continued development of these marginal assets. The authors will detail the development of the unique micro-bubble technology and its successful application in a mature and depleted field in Mexico.
Introduction
When formation pressures are drawn down, the type and severity of critical operating issues are exacerbated with few solution options. When taken in combination, the operator is often faced with substantially increased costs and risk, particularly where HSE issues are concerned.
The problems associated with the pressure variances encountered in mature fields, coupled with the limitations of conventional fluid and equipment technologies to properly provide an equitable solution, have driven the need for a new approach to drilling and workover operations within depleted reservoirs in mature fields.
One of the latest approaches to this dilemma is the use of the aphrons drilling fluid technology. In hundreds of wells in diversified applications worldwide, the aphron technology has proven to be a viable solution within these difficult parameters.
Field experience has shown that even annular pressures exceeding depleted reservoir pressures by "several thousand" psi has not hindered the creation of the micro-environment seal and mitigating invasion. This feature alone has allowed operators to eliminate casing strings, safely workover highly depleted wells and even drill into normally pressure plays on wells with existing depleted production. All of this has been affected without compromising production. Because the seal is internal to the reservoir (Fig. 1), conditions for differential sticking do not exist. In many parts of the world, this feature has successfully enabled high-angle and horizontal well construction of highly depleted reservoirs using conventional equipment. No other fluids technology in the industry enables these types of operations. Since standard wellsite fluid mixing equipment is all that is required to employ the technology, it is highly compatible with normal wellsite operations. As a drilling, completion or workover fluid, the aphron technology functions as the backbone of the operation. Serving as the bridging technology between the difficult operating parameters of mature fields and the limits of conventional equipment when employed to develop these depleted reservoirs, the aphron technology offers a solution for prolonging the economic life of these assets.
Statement of Theory and Definitions
Aphrons1 (micro-bubbles) are incorporated into a specifically engineered base fluid2 to aid in mitigating losses in depleted / highly permeable zones. These micro-bubbles differ significantly from aerated fluids and foams. Aphrons do not coalesce. Upon entering the lower-pressure region within a depleted formation, aphrons remain discrete, yet will agglomerate to create a stable, but easily removed, internal seal called a "micro-environment".3 Because affecting this seal requires a higher annular pressure than that of the reservoir, it readily cleans up with reservoir flow-back as production is initiated.
Synergies between the various features of the technology serve to minimize fluid invasion. The easily engineered high-LSRV (low-shear-rate viscosity) properties of the base fluid are achieved with high-yield, stress-shear-thinning (HYSST) polymers. This promotes design capabilities that are well suited for optimizing diversion of annular pressures away from the depleted formations, thus minimizing whole fluid invasion. The surfactant package and the associated "meniscus-wrapping theory"1,4 provide additional invasion control. The "energized-environment"4,5 associated with aphrons employed to form the internal "micro-environment" solids-free seal and the resultant localized increase in LSRV also contributes to mitigating invasion.
