Drilling fluid systems resulting in formation pressures between the equivalent of water and the equivalent of air would be useful, but have been difficult to develop. Since the successful field trial in January 2008 (Green, Minnett, Cremer 2008), these "middleweight" drilling fluid systems have been further developed. The original system has been refined, and a second approach has been prepared for field trial. The original system is generally referred to as the low pressure system, since air is mixed with liquid and then flows into the drilling rig mud pump. The second newer system is generally referred to as the high pressure system, because air is mixed with liquid after the liquid has passed through the drilling rig mud pump, just before entering the standpipe.
The drilling fluid is similar for both the low and the high pressure systems. It contains a small amount of polymer, clay and shale inhibitors and an air dispersant. The combination forms a solids free fluid that cleans the hole, keeps the formations stable, lowers equivalent circulating densities compared to conventional fluids, and allows the injected air to readily break out on return to surface. The fluid is not intended to form a filter cake, since filter cakes may result in diminished rates of penetration, and certainly result in higher than necessary pump pressures and equivalent circulating densities.
The low pressure system mixes air in a solids free drilling fluid just before entering the drilling rig mud pump. The first field trial showed that adequate pressure can be easily maintained while mixing air and liquid so that the drilling rig mud pumps can perform efficiently. The first field trial also showed that very small amounts of air volume are required to reduce the equivalent density of the fluid.
The high pressure system injects air into the fluid after it has passed through the mud pump. Higher operating pressures are required, and higher volumes of air can be injected into the fluid. The high pressure system may allow lower equivalent densities than the low pressure system.
Standard drilling practice shows that the pressure exerted on the formation should not exceed the formation fracture pressure. This has been simple enough to follow when the formation fracture pressure has been higher than the pressure gradient of water. When formations are drilled that are structurally weak or have been depleted of pressure or both, conventional fluids heavier than water have caused the formation to fail. The structural weakness may be due to a lack of consolidation such as in deepwater applications that begin with significant hydrostatic at the mud line, or may be due to leaching of cementation over time. There are several circumstances that result in seepage, lost circulation, stuck pipe, along with a much greater expenditure in time and money than planned for. Conventional practice to cure the problem of low fracture gradient has been to seal off the formation with lost circulation materials, cement, cross link polymer treatments, or at least strengthen the near well bore region with hoop stress treatments. The goal of these treatments has been to enable the formation to withstand the pressure gradient of water, plus some amount of equivalent circulating density (ECD). A second alternative has been developed to drill without returns to surface, such as mudcap drilling or simply drilling blind, hoping the drill solids are going into the formation somewhere, instead of falling around the drill string, and sticking the pipe. The third alternative has been to drill with air, mist or foam, which will produce pressure gradients as low as two pounds per gallon equivalent, sometimes less. All of these approaches can be expensive and do not always succeed. This paper documents ongoing work developing two methods for the mixing of air (or other gas) and solids free drilling fluid to produce a "middleweight" fluid, with a pressure equivalent in the range of four to eight pounds per gallon. This middleweight approach will avoid the problems of using heavier fluids to drill weak formations, without the expense of a complete air or foam drilling system, which many times has been used to reduce the pressure far lower than actually required.