Environmentally Acceptable Water-Base Mud Can Prevent Shale Hydration and Maintain Borehole Stability
- J.P. Simpson (O'Brien-Goins-Simpson & Assocs. Inc.) | T.O. Walker (O'Brien-Goins-Simpson & Assocs. Inc.) | G.Z. Jiang (Consultant)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1995
- Document Type
- Journal Paper
- 242 - 249
- 1995. Society of Petroleum Engineers
- 4.3.1 Hydrates, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 2.4.5 Gravel pack design & evaluation, 1.8 Formation Damage, 5.1 Reservoir Characterisation, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 2.4.3 Sand/Solids Control, 6.5.4 Naturally Occurring Radioactive Materials, 1.10 Drilling Equipment, 1.2.3 Rock properties, 1.11 Drilling Fluids and Materials, 1.6.6 Directional Drilling, 1.14 Casing and Cementing
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This paper presents laboratory and field results showing that an environmentally acceptable water-based mud (WBM) can be formulated to act like an oil-based mud (OBM) in preventing hydration, pore-pressure increase, and weakening of shale by effectively developing sufficient osmotic force to offset the hydraulic forces acting to cause water flux into the shale. We also show that this methyl glucoside system has other performance characteristics similar to OBM's, such as lubricity, filtration control, and tolerance for common WBM contaminants, thus meeting the needs for drilling high-angle or even extended-reach wells.
In spite of years of study, problems of borehole instability continue to be a major factor in the cost of petroleum drilling, logging, and cementing. Today's efforts to drill extended-reach wells are especially affected, with borehole instability often preventing the objective from being attained when currently available WBM's are used. With the use of OBM becoming more restricted by environmental regulations, there is a great need for an environmentally acceptable WBM system that can provide borehole stability, filtration control, and lubricity.
Thorough studies of borehole instability have been made from a rock mechanics perspective. Such studies show that failure can be predicted on the basis of measurements of properties of the rock, observations of in-situ stresses, and estimates of pore pressure, along with knowledge of hole inclination, hole direction, and mud pressure. The predictions usually indicate that there is a maximum mud weight that can be used without tensile failure causing lost circulation, as well as both a minimum and maximum mud weight that will avoid borehole instability from compressive failure or shear displacement. When OBM is used for drilling, the predictions are usually found to be valid and the well can be drilled without borehole-instability problems. Such is often not the case when WBM's are used for directional drilling through shales. If shale formations can be drilled and cased off quickly, there may be no serious problems. However, if shales are left exposed for very long (by design or because of equipment failures or human error), borehole instability is probable because time-related shale hydration reduces the strength of the rock and changes other related properties.
This paper will present data demonstrating that OBM can protect against borehole instability by preventing hydration, build-up of pore pressure, and weakening of the shale. In contrast, we show that typical "inhibitive" WBM's allow water flux into shale, increase in pore pressure, and changes in properties of the shale near the borehole surface. Mechanisms will be proposed to explain the differences in performance of currently used OBM's and WBM's. Data will then be presented showing that an environmentally acceptable WBM can be formulated to act like an OBM in avoiding water flux into shale, pore pressure increase, and weakening of shale, with corresponding maintenance of borehole stability. We also show that this methyl glucoside system has other performance characteristics similar to OBM's, such as lubricity, filtration control, and tolerance for common WBM contaminants. Such performance is needed to avoid excessive torque, drag, and fill and the sticking of pipe and tools often associated with extended-reach drilling.
|File Size||1 MB||Number of Pages||8|