1996 · 76 citations · 0 references
SPE members Abstract New and improved water-based drilling fluids based on soluble silicates have been developed and applied in the field in a unique collaboration between BW Mud, Mobil NSL, BP Exploration and Shell Research. Silicate-based muds are introduced as superior fluids for drilling troublesome formations like intact and (micro-)fractured shales and chalks. In addition, these inorganic systems are environmentally friendly and inexpensive. Details are presented on the wellbore stabilisation mechanism of silicate-based muds and their drilling fluid engineering. In addition, field trial results are presented which demonstrate the excellent cuttings- and wellbore-stabilising capacity of these exciting new mud systems. General Introduction Oil-based muds (OBMs) and synthetic fluids are generally regarded as the most reliable options for drilling troublesome formations and difficult well-paths. Apparently, these fluids succeed where conventional water-based muds (WBMs) fail. However, increasing environmental and economical pressures strongly favour the replacement of OBMs and synthetics by improved WBMs. Any replacement WBM should ideally offer–Good environmental compatibility. The new WBM should best be formulated in anticipation' of more restrictive environmental legislation and therefore show exceptionally low toxicity, biodegradability and no bio-accumulation, and be harmless when introduced into the environment.–Cost-efficiency. Full-containment, clean-up, alternative disposal etc. and high base fluid cost add to the expense of OBMs and synthetics. Inexpensive, benign WBMs that can be disposed of after use could offer strong reductions in direct mud cost and in solids- and fluid-processing cost.–Technical competence. To be truly cost-effective, new WBMs should give a drilling performance (i.e. ROP, dilution rates, reduction of mud-related trouble time etc.) similar to OBMs and synthetics. These considerations have guided the development of new silicate-based WBMs. The drilling industry, however, has been using such mud systems before, as shown below. History of silicate-based drilling fluids Drilling muds based on soluble silicates for the control of "heaving shale" (i.e. shale that sloughs into the wellbore in excessive quantities, interfering with drilling progress) were first introduced by Garrison, Baker and Vietti in the 1930's. These muds, based on high concentrations of soluble silicates (i.e. 20-50% v/v) were successful in drilling over 100 wells in the Gulf Coast Area, but were difficult to control because of their high rheologies. They were replaced in the mid-1940's by lime-quebracho muds which were thought to provide adequate shale control and much easier rheological control. Silicate-based muds were re-introduced in the late 1960's by Darley, who had found that most shales could also be stabilised with more dilute solutions (less than 20% v/v, typically 5-10% v/v) of sodium- and potassium silicates, the latter systems being the more effective. Adding simple salts (e.g. KCl, K2CO3, NaCl) or non-ionic solutes (e.g. glycerol, ethylene glycol) contributed significantly to shale stability. This additional stabilising effect was attributed to shale dehydration through an osmotic gradient generated by the solutes. Three wells were drilled using the formulations suggested by Darley. These trials reflected the superior wellbore-stabilising properties of the new silicate-based muds, but did unfortunately not herald their breakthrough and acceptance in the field. P. 189