Reducing Well Cost by Gravel Packing in Nonaqueous Fluid

Abstract

Abstract As operators continue to develop prospects in expensive, deep water environments, delivery of high rate, high capacity, long life wells become essential for economic success. Often, operators are drilling high angle, open hole wells to maximize production by capturing reserves from several sand packages in one wellbore. In many of today's deepwater applications, hole stability and shale inhibition are keys to a successful open hole completion. Nonaqueous fluid (NAF) is the preferred fluid system to optimize drilling of the open hole section, but hole stability can be problematic during displacement operations prior to gravel packing. This paper describes a gravel packing technique that allows the operator to run the gravel pack screens in NAF, gravel pack the well and displace the casing to brine, all in one trip. Running the screens in NAF significantly increases the probability of successful screen installation and the end result is consistent, cost effective delivery of high rate, low skin completions. Introduction Angola Block 15 is located 140 kilometers off the west coast of Angola, in 1200m of water. (Figure 1) Early exploration drilling and core analysis suggested NAF, a low toxicity mineral based mud, would be the fluid of choice to best drill the Block 15 formations. While drilling the openhole section, several sand and shale sequences would be encountered. The shales are very sensitive to brines and exhibit severe swelling. Testing indicated that water-based fluids did not provide sufficient inhibition to allow drilling and completing shales between the productive intervals. Selecting a suitable NAF fluid would not only inhibit the shales, preventing shale swelling, but the NAF would also reduce the torque and drag while drilling and yield lower equivalent circulating densities (ECD). Reducing ECDs in this environment is very important due to the narrow margin, 0.5 pounds per gallon (ppg), between circulating and fracture pressure. Recognizing the significant benefits of drilling with NAF, the decision was made to drill the productive interval with NAF to increase the probability of successfully drilling the hole to the desired total depth (TD). The unconsolidated nature of deepwater formations often requires sand control during the producing life of the well. Operators generally have two options for open hole completions, stand alone screens (SAS) or gravel packing. Although SAS is the easiest and least expensive sand control option, some formations cannot be successfully produced utilizing SAS due to non-uniform formation sand sorting. Core samples from exploration wells in Angola Block 15, indicate the rock is unconsolidated and sand control is required. Sieve analysis performed on core samples from exploration and appraisal wells show a wide range in the uniformity coefficient (D40/D90) of 3–30 and sub-44 micron particles (5%-50%). The need for sand control is also supported by exploration well test results where both production of formation sand during the test and plugged gravel pack screens at low drawdown pressures (50 -200 psi) were experienced. Given the sieve analysis and exploration well testing observations, SAS screens would not be acceptable under these conditions. An openhole gravel pack was chosen as the optimum sand control method. Conventional gravel packing practices have required the entire cased wellbore and openhole to be displaced from the mud system to filtered brine before running the screens. Managing wellbore stability during this displacement phase of an openhole completion has proven to be a challenge especially when the displacement involves NAF to filtered brine. Fluid compatibility issues and formation stability problems associated with brine displacements have been experienced in two other company developments, offshore West Africa and deepwater Gulf of Mexico (GOM). Hoover/Diana is a deepwater GOM development where horizontal openhole gravel packs were employed. Ten horizontal wells were drilled with a brine based mud. A solids free pill was typically placed in the openhole section prior to running gravel pack screens, then displaced after setting the gravel pack packer. On one occasion, the openhole was displaced to a brine, rather than a solids free pill, prior to running the gravel pack screens. The screens were stuck 400' from TD, leaving 1/3 of the productive interval exposed with no form of sand control.