Summary 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. Introduction 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.

Further development of the fractured Natih field requires a good understaliding of the historical contributiolis from gas/oil gravity drainage and water/oil displacement. A history match of the field has been made using a fractured reservoir simulator, which apart from dual porosity, can model dual permeability and block-blook interaction. Several studies aimed at describing the reservoir in detail were required before simulation could start. The history match shows that gas/oil gravity drainage has been about twice as effective as water/oil displacement. This figure is in good agreement with the gas and water sweep data derived from recent gas saturation and water saturation measurements in the field. Prediction runs show that promoting gas/oil gravity drainage by lowering the fracture oil rim is an attractive way to further develop the Natih field.

CWwt 1S90, fADC/SPE Drilliw Confw-This pawr was prepe red fw Vesentation at the 1S9S tADC/SPE Drilliw CMferw held in Dallas, Texas, 3-6 Mti 1S9S. This paper ws selected for Wesenlatiw by an lADC/SPE Pregram Commitiee follewing review ef informalien contaln~h an abstrad submRted by the aufher(a). Centents of the peper, aa preaentad, have not baen reviewed by the Intamafimal Aaseciatiin of Drilling Centractora or the Society ef Petroleum Engineers and are subject to correefion by the a-s) w m~eriali aS WS~I~, d-~PSOEfllY M* any~a~~~he WC w SPE, tkir efficera, er members Paper8 preeanted qt the lAOC/SPE mear~s are suwect to publication raviaw by Edioriel Cmmittees cf the IADC and SPE. Elactronk reprcd~err, distriwm, w atore~ef any part ef this w fw eemm~ial mwa~~fhe Wffitan mnsenf ef the Seciety of Patrolw Eng*ra is @ibifed. Permission to mm h print is rastricfed to an abstract of net mere then W words: illuatratione may net be copied The abstract must eciWain -spiwous acknawtedgment & where and by whern the paper was presented. Wrte Librarian, SPE, PO. Box 83W3e, RWrdeon,~7~3-3e3S, U.SA., fax 01-972-9S2-9435. AbstractShale instability problems when using water-base drilling fluids have remained unresolved for decades because of a lack of knowledge and understanding of the shale hydration mechanisms. The industry has relied upon hydrocarbon-base drilling fluids for combating shale problems, but misconceptions have kept even those fluids from being utilized to their fullest advantage.With the use of hydrocarbon-base fluids now being curtailed because of environmental concerns, costs due to shale problems could escalate.The understanding of shale instability problems has been hindered by inadequate laboratory means of simulating contact of drilling fluid and shale under downhole conditions of stress and temperature. To address this situation Gas Research Institute has conducted a project in which laboratory equipment and procedures were developed to permit preserved specimens of downhole shale (cored in hydrocarbon-base mud) to be restored to in situ axial stress, horizontal stress, pore pressure and temperature prior to being drilled at a selected borehole pressure.Provisions were made for measurement of fluid transport in either direction between the circulating drilling fluid and the shale during an extended period of exposure. The borehole pressure was then reduced incrementally to observe for borehole failure and obtain a measure of effect of the &llling fluid on the relative stability of the shale.The above procedures have been used to study a wellknown troublesome Cretaceus shale cored using oil-base mud at a depth of about 5,500 ft in Block 4 of the U.K. sector of the North Sea.~is paper presents data showing that the aqueous activity of either a water-base or hydrocarbon-base emulsion drilling fluid can be adjusted to develop osmotic pressure that will cause water to enter or be extracted horn a low-permeability shale. The hydraulic differential between the borehole pressure and far-field shale pore pressure ...

Laboratory data are presented showing that lime muds utilizing a polysaccharide deflocculant are very effective in combating dispersion of shale particles when compared to several commonly used types of water based muds. Similar studies using Wyoming bentonite particles are reported snowing that potassium hydroxide results in less clay dispersion than sodium hydroxide when used for alkalinity control of a lime mud deflocculated with polysaccharide. While the potassium is shown to be reactive with clay particles, the presence of lime in the mud leaves more potassium ion available to react with shale exposed in the wellbore and protect against borehole instability. Field results are reported showing that potassium ion concentrations of 1,000 to 4,000 mg/L have been adequate in potassium/lime muds containing polysaccharide deflocculant to provide good borehole stability and low mud maintenance costs.

SPE Members Abstract A recently developed, environmentally safe, water-based drilling fluid has been given its first field trials. The successful field tests have shown that the fluid is indeed very shale stabilizing, has the ability to solve some mud related drilling problems, is easy to formulate and maintain, and is non-hazardous and environmentally safe. These results have corroborated the laboratory testing which had shown that the fluid stabilizes shales by the same mechanism as does oil-based muds. The drilling fluid, which is based on methylglucoside (MEG), thus has the potential to replace oil-based drilling fluids in many operational areas. The use of this drilling fluid could reduce or eliminate costly disposal of oil contaminated drilled cuttings, minimize health and safety concerns, and minimize environmental effects. Introduction Oil-based drilling fluids are used routinely in many operational areas. These applications are normally in areas where the drilling situation requires the advantages provided by the excellent performance of oil-based drilling fluids. However, in many of these areas the use of oil-based drilling fluids is cause for increasing concern due to environmental restrictions and disposal costs. Water-based drilling fluids that can give oil-mud like performance are needed for use in these situations. Any water-based replacement fluid must possess those characteristics that make the oil-based fluid a good choice for the given application. Oil-based drilling fluids can provide superb borehole stability, are highly resistant to contamination, and are stable under high temperature conditions. The use of oil-based muds also can give excellent drilling performance when used in conjunction with PDC bits. The water-based mud must replicate the performance of the oil-based fluid. A new water-based drilling fluid system has been developed that has an excellent chance of replacing oil-based muds in many applications. The fluid is based on methylglucoside (MEG). As indicated by its structure as shown in Figure 1, methylglucoside is a chemical derivative of glucose. Methylglucoside is supplied as a liquid containing 70% solids. As supplied it contains about equal portions of the alpha and beta forms of methylglucoside. P. 605