An Offshore Benign WBM System Can Prevent Hard, Brittle Shale Instability

Abstract

In the Weizhou Southwest oilfields, drilling delays and suspension of wells prior to reaching the targets due to wellbore instability had occurred frequently. The hard, brittle shale played a problematic role. Conventional water-based drilling fluids did not conquer the problematic formation due to intrinsic performance deficiencies. Though oil-based drilling fluids are routinely preferred in more technically demanding applications, they are cause for increasing concern due to offshore environmental restrictions and expensive disposal costs. An environmentally acceptable water-based drilling fluid was developed to challenge the problematic formation based on the combination of methylglucoside-silicate concept. It stabilized the reactive shale by the same mechanism as oil-based drilling fluid in preventing shale hydration, pore pressure increase, and weakening of shale by effectively developing sufficient osmotic force to offset hydraulic and chemical forces acting to cause filtration flux into the hard, brittle shale. A field trial was initiated on the CNOOC 931 platform. The data from the pilot well showed that the novel drilling fluid exhibited excellent inhibition and lubricity that approached or even exceeded oil-based fluids.

Keywords:

• drilling fluid
• environmentally benign
• hard
• brittle shale
• hole instability
• ideal packing theory
• methylglucoside

Notes

^a 1, formulation shown in Table 1; 2, No. 1 + 4% (w/v) Rev-Dust; 3, No. 1 + 6% (w/v) Rev-Dust; 4, No. 1 + 8% (w/v) Rev-Dust.

^a Percentage of return permeability is the value obtained by dividing the final permeability to oil of the core by the initial permeability to oil and then times 100 for the whole core.

^b The calculation of breakthrough pressure: the maximum pressure minus the stable pressure when determining the final permeability. It takes into account the changes in flow rate by normalizing pressure to flow rate.