Gas migration, inter zonal water flows, and cementing low frac gradient wells remain some of the biggest completion problems facing the oil and gas industry today. These are zone isolation issues for which operators used to count on the primary cement job alone to address. Operators and cement service companies valiantly tried many techniques to improve zone isolation including multiple scratchers and centralizers, improved mixing systems, altering flow rates, stage systems, reduced weight and high early gel strength cements and additives for fluid loss control. Still, operators were frequently required to conduct costly remedial cementing operations to correct zone isolation problems (microannulus, channeling, hydrocarbon contamination of cement, thief zone isolation, etc.), after primary cementing.
Inflatable casing packers (inflatables) were designed to address these zone isolation problems and were first run in the early 1960s. They have since become a standard part of the casing string for many operators as a technical addition to conventional cementing. Inflatables have an external element that is inflated with cement, mud, brine or produced fluid to provide a positive mechanical seal between the outside of the casing and the wellbore. Inflatables can be placed above or near the critical zone in question or in the lap of the liner to help eliminate these costly zone isolation problems.
Zone isolation in a well does not require that the annulus be completely filled with cement. An inflatable effectively seals the annulus resulting in a reduced or eliminated need for a cement column. Using an inflatable within the cement column results in a non-channeled section of cement immediately below the element. This phenomenon is the result of the inflate element's expansion in the cement column. The net result of these is a superior completion and a cost saving and, since the cost of the packer, in most cases, is substantially lower than the cost of cementing materials, equipment and service.
Most horizontal wells today are produced via open hole techniques which do not easily allow for control of sections that are producing water or unwanted gas. The lack of isolation also limits the ability to clean up the hole and conduct remedial stimulation prior to production. Other factors such as changing permeability and variations in reservoir pressure can result in production losses. Inflatables are increasingly used to isolate sections of the horizontal hole to overcome these limitations.
Large scale manufacture, sale and service of inflatables started in the early 1980s with several companies producing a variety of sizes, ranging from 2 3/8 inch to 20 inch, and lengths ranging from three feet to 40 feet. How well have these inflatables performed? Two initiatives are underway to explore the application of inflatables.
In the early 1990s British Petroleum recognized that with the increasing number of horizontal wells being drilled around the world and with the take off of multi-lateral wells, there was a strong case for examining the effectiveness of inflatables. In 1994 BP Exploration Technology Provision completed an investigation into the effectiveness of inflatables based upon the combined worldwide experience of BP and ARCO Alaska.
