Results are presented from laboratory drilling studies of two different hard shales similar to those found in some areas ofthe North Sea. Two laboratory drilling machines with milled-tooth bits were used. Pore pressure was measured in all the experiments, and in some it was reduced to several megapascals below bottornhole pressure (BHP). Pore-pressure reduction was achieved by allowing the pore fluid to drain out of the shale while external stresses were maintained constant. The influence of BHP was studied for values from 3 to 33 MPa [435 to 4,786 psi]. The differential pressure, which equals BHP minus pore pressure, ranged from near balance to > 10 MPa [> 1,450 psi]. BHP was found to have a strong influence on drilling response, but differential pressure did not, contrary to conventional wisdom. The results are compared with other drilling experiments and single-cutter experiments in the literature.

Summary Traditional pore-pressure interpretations in tertiary under compacted shaleshave been based on empirical relationships between a particular measurement(such as resistivity) and pore pressure in pounds per gallon. It is well-known, however, that the pressure in pounds per gallon. It is well-known, however, that the measurements are responding to the "excess" porosity in theshale rather than to the pore pressure directly. A new technique is illustratedin which all available measurements are first characterized in terms thisexcess porosity and lithology with measurement-response equations. This allowsa mathematical minimization technique to solve simultaneously the variousmeasurement-response equations for this porosity and lithology on afoot-by-foot basis. A conventional compaction porosity/effective-stress modelis then used to determine the additional pore pressure caused by this excessporosity. The result is a single pore-pressure estimate that is independent ofthe number of measurements and that has an accuracy that improves with thetotal number of measurements used in the interpretation. The interpretation canbe performed at the wellsite in real time by us of rate of penetration (ROP), measurement while drilling (MWD), and logging while drilling (LWD) measurementsor after drilling by us of these measurements in conjunction with wirelinemeasurements. Introduction The traditional empirical relationships that have been developed between aparticular measurement and pressure are usually displayed as a series of linesor trends that the user places over the measurement. Excursion of a measurementfrom the normal hydrostatic trend is then interpreted as abnormal pressure andis automatically scaled in terms of pore pressure in pounds per gallon, whichis the equivalent mud density necessary to balance the formation porepressures. This method has been used successfully in the U.S. gulf coastpressures. This method has been used successfully in the U.S. gulf coast byskilled interpreters. This method has several shortcomings, however. It canlead to as many pore-pressure estimates as there are measurements to evaluate;the empirical relationships are locally confined and are not generallyapplicable outside the U.S. gulf coast; and lithological variations in theshales and their effects on the measurements are not accounted for, causingadditional uncertainties in the pore-pressure estimates. The use of ROP forpore pressure (the D exponent 1) has proved difficult to apply because ofvariations in lithology, bit wear state surface-to-downhole weight transferefficiency, and bit types. A new approach first corrects ROP for bit wear byuse of downhole measurements of weight and torque. ROP is also normalized forbit type, downhole weight, and surface revolutions per minute. This produces anapparent formation strength that is. a measurement of produces an apparentformation strength that is. a measurement of the rock failure resistance to thebit teeth. Previous attempts to characterize and quantify this strength interms of laboratory-measured rock properties have been largely unsuccessfulbecause the bit teeth do not fail the formation in the same manner as alaboratory load cell. In our new approach, the formation strength ischaracterized by traditional interpretation volumetrics. A thorough analysis ofthe formation strength shows that it is a strong function of the lithology andeffective porosity of the formation. Evidence of the nature of theserelationships is found in data where the classic shaly sand "boomerang"seen on the neutron-density crossplot is also seen on aformation-strength/gamma ray crossplot. This unique analysis allows theformation strength to be interpreted in a manner consistent with conventionallog analysis where clay, matrix, and effective porosity volumes are derived. The formation strength is also quantified in terms of the in-situ stress stateof the formation, which is largely a function of the mud pressure and near-bitpore pressure. pore pressure. Combining the formation-strength measurement withother MWD and/or LWD measurements allows the pore pressure, lithology, effective porosity, and saturation of the formation to be computed in real timeduring drilling and allows decisions to be made that promote safer and moreefficient drilling. promote safer and more efficient drilling. Technique Interpretation Model. A formation can be described by the volumes shown in Fig. 1, which represent the majority of the constituents of sedimentary rocks. The volumes determined by the interpretation program described in this paperare illustrated on the right side of Fig. 1. When the interpretation programhas determined that a shale is present, matrix (usually quartz), wet clay, overpressure porosity, and effective porosity volumes are solved for. Theporosity, and effective porosity volumes are solved for. The saturation is setequal to unity in shales. When the program has determined that porous sands arepresent, matrix, wet clay, effective porosity, and water saturation are solvedfor. The pore pressure porosity, and water saturation are solved for. The porepressure computed in the shale above the sand is considered to apply to thesand interval also. Overpressure Porosity to Pressure Characterization. The effective stress 2and equivalent depths 3 concepts are illustrated in Fig. 2. In a normalpressure environment (right side of Fig. 2), the rock grains are and water isexpelled as the overburden stress The water contained shales consists of waterbound to the clays and nonbound or "free" water contained within thepore space. The water expelled during compaction consists pore space. The waterexpelled during compaction consists predominantly of the free water. Conventional log interpretation predominantly of the free water. Conventionallog interpretation nomenclature defines an effective porosity consisting ofthis free water and any hydrocarbons. Although "effective porosity" maynot be the best term to describe shale porosity, it will be used throughout thefollowing discussions in keeping with normal usage.