Invert Emulsion Fluids for Drilling Through Narrow Hydraulic Windows

Abstract

In mature fields the difference between the pore pressure and fracture pressure, expressed as the hydraulic window, is reduced. Underbalanced drilling and the use of low-viscosity drilling fluids are but a few of the many approaches that have addressed this challenge. While low-viscosity fluids severely reduce frictional pressure loss when compared to more viscous fluids, there is a limit to how much viscosity can be reduced before conventional weight material begins to settle. This paper describes the development and application of novel technology that has resulted in a ten-fold reduction in the particle size of the weighting agent. With this development, invert emulsion drilling fluids can be designed with low viscosity with minimal settling potential of the weight material. The authors will explain in detail the first field applications of this polymer-coated, micron-sized weighting agent in an oil-based drilling fluid. The system was used to drill two wells offshore Norway. These reservoir wells included an 8 1/2-in. section and a 5 7/8-in. thru-tubing section. This novel drilling fluid technology delivers a low-rheology, low-sag fluid offering a number of performance benefits over conventional technology. Performance data from offset wells will be presented that show the benefits of this unique approach. Equivalent circulating densities (ECD) and torque values were significantly lower than comparable wells and no instances of particle settlement were observed. Introduction The Statfjord Field was discovered in 1973 and declared commercial in August 1974, with production startup in 1979. The field spans approximately 25 km by 4 km, and is the largest producing oil field in Europe. Statfjord is located in the Tampen Spur area, in the northern portion of the Viking Graben, and straddles the border between the Norwegian and UK sectors. The field is developed by three fully integrated Condeep concrete platforms. Production is from the Brent, Dunlin, and Statfjord reservoirs, with Brent and Statfjord being the main reservoirs. Cumulative oil production by end of 2003 is forecasted to 626 million Sm3, giving a recovery factor of 63%. The aggressive drilling campaign necessary to achieve such a high recovery is described by Hansen, et al.1 Development of the field has required drilling of long and complex wells, often in reservoir compartments with high pressure depletion. Good hole cleaning and ECD have been and still are main focus areas for achieving a successful drilling operation. Introducing annular pressure-while-drilling tools in the mid 90's highly increased the understanding of hydraulic behaviour in the drilling process.2 However, introduction of Through-Tubing-Reservoir-Drilling (TTRD), possible future ERD wells, and several lost circulation events in the later years, have pinpointed the need for a highly inhibitive drilling fluid system exhibiting a low ECD, low friction and low sag potential. A drilling fluid with these combined properties will also be of key importance if the planned pressure blowdown of the reservoirs in the field is being carried out, as this will narrow the hydraulic window further. In case of pressure blowdown, there will also be a need for openhole sand screens, and the drilling fluid must preferably be "screen friendly". The geological anomalies of Statfjord and similarly mature fields worldwide prompted the initiation of a weighting agent research project (WARP). Conceptually, the foundation of the program was to employ ultra-fine particles of a weighting agent that have been polymer coated to provide a number of technical benefits in drilling fluids. The weighting agent, in this case barite, is milled in an enhanced mineral oil using high-performance milling technology. In this manufacturing process as the barite particles are milled, the new surfaces that are continuously exposed are coated with the special polymer additive. This coating provides effective oil-wetting of the barite weighting agent to produce stable high-solids, high-density and less viscous slurries.

Effective Ways to Avoid Barite Sag and Technologies to Predict Sag in HPHT and Deviated Wells

“…Afterwards, the slurry-weighting agent can be used to formulate high performance drilling fluids with minimal sag potential. Despite the ultra-fine particle size of the polymer-coated barite, drilling fluids can be formulated from the stable slurry with low rheologies, thereby increasing the potential for improved ECD management (Fimreite 2004). …”

Improve Drilling Performance by Controlling Risk and Reducing Non-Productive Time in a Challenging High-Temperature, High-Pressure Well

“…The sag stability of the TMB OBM system, which is a high density and stable liquid suspension of micronized weight material 4,5,6 was tested in the laboratory with promising results after 16 and 160 hr (Table 6). To prepare the liquid suspension of micronized weight material, barite is milled to a D 50 of ±1 micron and when the milling process is complete, the barite is no longer in a dry state as liquid chemicals are added to turn this into slurry.…”

Managed-Pressure Drilling; Techniques and Options for Improving Efficiency, Operability, and Well Safety in Subsea TTRD

“…• Low ECD (achieved through low viscosity) • Low solids settling tendency (achieved through high viscosity) Two major drilling fluid service companies have solved these requirements differently, but the results are the same; by applying weight material with small particle size both rheology and sagging tendency have been improved compared with conventional mud systems 6,7 . Formate mud, where density is achieved through soluble salts and not through solids is an attractive alternative but its high cost may limit its application 8 .…”