As to the strong sensitivity of shale to drilling fluids, drilling in shale gas wells is still challenging today. People have long hoped to improve the anti-sloughing ability of drilling fluids mainly by designing the proper density, sealing ability and inhibition ability. However there has not been a quantitative criterion to evaluate these parameters or to define "proper". This could lead to high cost of excessive additives, or wellbore instability due to the scarcity of additives.
In this paper, some essential parameters of drilling fluids, which are mud density, sealing ability and inhibition ability, were quantitatively developed based on rock mechanics theories and laboratory survey. The time-dependent fluid-solid-chemistry coupling model for wellbore stability was established to analyze the near-wellbore stress distribution. A dynamic core flow apparatus was used to investigate the shale-fluid interaction and fluid pressure transmission under the simulated HTHP downhole conditions. Shale mechanical parameters were studied under HTHP conditions by applying a true triaxial compression cell.
In the paper, the influences of mud sealing and inhibition ability on wellbore stability were studied. The research results show that the sealing ability and inhibition ability of drilling fluids do play an important role in enhancing wellbore stability. The volume of drilling fluids invading into shale was defined as the "sealing coefficient" to quantify the sealing ability of drilling fluids. The mechanical strength of shale exposed to the drilling fluids for some time was defined as the "inhibition coefficient" to quantify the inhibition ability of drilling fluids. The quantitative cross plot among mud density, sealing coefficient and inhibition coefficient was obtained from fluid-solid-chemistry model according to the field required stability duration and the borehole enlargement ratio. Practically, the combinations of mud additives could be applied to fit the quantitative diagram regarding the cost and convenience.
The field application results indicated that it is reliable to quantitatively design drilling fluids based on the fluid-solid-chemistry coupling theory. A new idea and way of designing drilling fluids and maintaining shale wellbore stability was introduced.