Use of Lightweight Solid Additives To Reduce the Weight of Drilling Fluid in the Riser

Abstract

This paper describes the first phases of development of a drilling system for offshore wells based on the use of lightweight solid additives (LWSA) to reduce the density of drilling fluid within the riser above the seafloor. Equipment and procedures were tested to pump LWSA down to the bottom of the riser without damage, separate them from the mud after the fluid mixture returns to the surface, recycle LWSA for immediate re-use, as well as several other problems. Initial developments and tests are described that successfully addresses many of these issues. In offshore wells, particularly in deep water, the problem of maintaining a safe range of mud weights is compounded by the additional pressure that drilling mud in the riser exerts on the formation. If the effective weight of mud in the riser can be reduced, the range of safe mud weights is effectively widened at the formation, resulting in fewer casing strings and safer operations. Mud with high concentrations of LWSA might be pumped down from the surface through special lines and then injected into the riser at the mud line. The mixture of mud and LWSA flowing up the riser then weighs less than pure mud. LWSA were developed and tested as spheres produced from different materials including glass, composites, and epoxy resins. Tests showed that LWSA could withstand high pressures existing at the bottom of risers. LWSA were able to be separated from oilfield muds using conventional oilfield shale shakers and hydrocyclones. Muds with LWSA could be pumped with conventional mud pumps without excessive pressure losses. LWSA were recirculated many times with minimal breakage, demonstrating that they could survive in commercial operations. Significant cost savings will be possible in drilling offshore wells if remaining problems with LWSA can be addressed. In addition to reducing the number of casing strings required, drilling with LWSA would reduce tension load requirements on the riser and mud storage requirements on the drilling vessel, which will reduce the size of drillships or increase depth capability of existing vessels. Introduction Deep Water Drilling Problems. Problems are often encountered in deepwater wells with lost circulation and fluid influx. These issues increase costs and make deepwater drilling uneconomical in many areas. One major concern that exacerbates these fluid problems is maintaining wellbore annulus pressure above pore pressure so that the well does not "kick", and below fracture pressure so that the well does not hydraulically fracture and lose circulation. In deep water, pore and fracture pressure gradients are typically close together, making drilling very difficult. Figure 1 shows mud hydrostatic pressure gradients for offshore drilling. Because of the seawater column, and the unconsolidated nature of sediments near the seafloor, the pore pressure (A) and fracture pressure (B) curves are often close together, making it difficult to maintain wellbore annulus pressure between these curves.

Managed-Pressure Drilling: Using Model Predictive Control To Improve Pressure Control During Dual-Gradient Drilling

“…The term DGD includes several different drilling concepts whose common theme is that the pressure profile in the annulus is not linear but follows a piecewise linear or curved pattern because of the use of fluids of varying density along the annulus. Density variation can result from different dilution-based approaches, such as injection of lightweight solid additives to reduce the density, proposed by Maurer Technology (Cohen and Deskins 2006), and injection of lightweight fluids into the riser to lower the efficient fluid density above the injection point, proposed by Louisiana State University (Lopes and Bourgoyne 1997). Diversion-based concepts, where the mud returns do not travel through a riser, but are either dumped at the seafloor or circulated back to the rig through a separate return line, have been proposed by Deep Vision (Forrest et al 2001), SubSea MudLift Drilling Joint Industry Project (Schumacher et al 2002), Shell (Gonzalez 1998), AGR (Brown et al, 2007), and Ocean Riser Systems (Fossli and Sangesland 2006).…”

Automatic control of managed pressure drilling

“…The term DGD includes several different drilling concepts, where the common ground is that the pressure profile in the annulus is not linear, but follows a piecewise linear or curved profile, due to the use of fluids of varying density along the annulus. Density variation can be based on different dilution-based approaches, such injection of light-weight solid additives to reduce the density, proposed by Maurer Technology (Cohen & Deskins, 2006), and injection of light-weight fluids into the riser to lower the efficient fluid density above the injection point, proposed by Louisiana State University (Lopes & Bourgoyne, 1997). Diversion-based concepts, where the mud returns do not travel through a riser, but are either dumped at the seafloor or returned back to the rig through a separate return line have been proposed by Deep Vision (Forrest & Bailey, 2001), SubSea MudLift Drilling Joint Industry Project (Schumacher et al, 2002), Shell (Gonzalez, 1998), AGR (Brown et al, 2007), and Ocean Riser Systems (Fossli & Sangesland, 2006).…”