During laboratory drilling tests at simulated downhole conditions in shale, the performance of a polycrystalline-diamond-compact (PDq bit was measured over a wide range of rotational speeds covering rotary, positive-displacement-motor (PDM), and turbine drilling applications. Tests were conducted at a constant borehole pressure with a standard water-based mud. Rate of penetration (ROP), torque, and mechanical horsepower at the bit were determined as functions of rotational speed, bit weight, and flow rate. From the test results, the cost benefit of higher rotational speeds achieved with PDM and turbine drilling was examined in terms of break-even costs relative to rotary drilling.
Summary A combination of advanced Polycrystalline Diamond Compact (PDC) core bit technology and modified coring techniques has produced waterbase mud cores of high permeability sandstone with no mud filtrate invasion over two-thirds of the core's cross-section. Waterbase mud filtrate invasion while coring detrimentally affects water saturation and its distribution, residual oil saturation, and rock wettability which are used to calculate total reserves, movable oil, etc. Multimillion dollar decisions are based on these parameters which have a limited reliability due in part to mud filtrate invasion. This coring technique will eliminate mud filtrate invasion as a factor in these measurements. The majority of filtrate entering the core is generated by dynamic fluid loss from gage cutters in and near the throat of the core bit. Core invasion can be minimized by increased coring rate, reduced filtration area, increased bridging solids in the mud, and reduced contact time with gage cutters. This has been achieved by:reducing the number of cutters over the entire bit (increases depth of cut which can increase penetration rate);using a parabolic bit design (reduces the dynamic filtration area);using a low fluid loss mud (increases bridging solids);reducing the number of gage cutters (reduces contact time of gage cutters) andeliminating all throat diamonds (leaves mud cake intact). These bit design criteria are supported by laboratory coring tests and analysis. Three versions of these improved core bits have been tested in full scale laboratory coring operations. Bromide tracers have been used to evaluate the amount of mud filtrate contamination. These tests indicate invasion of filtrate can be limited to the outer three fourths of an inch (1.905 cm) of a four inch (1 0. 1 6 cm) diameter waterbase core at coring rates greater than 90 ft/hr (27.6 M/hr). The limited core invasion seen here is interpreted with a simple model incorporating the mechanisms of spurt-loss from PDC cutters in contact with the core and static filtration above that point. point. The conclusions reached from laboratory core invasion have been verified in field tests by coring sandstone oil reservoirs containing connate water saturation. This coring method has provided reservoir rock having in-situ rock wettability and water saturations unaffected by filtrate invasion. Introduction Filtrate invasion while coring has been a major factor affecting the validity of fluid saturations and laboratory measurements such as wettability and relative permeability. When pressure coring to measure oil saturation, tracers are often used to evaluate invasion. Filtrate invasion will decrease high initial oil saturations and in the most severe cases may even mobilize residual oil saturation. These effects may be minimized by increasing core diameter and formulating drilling muds with low spurt loss. High coring rate and decreased overbalance between the mud and the formation have also been identified as beneficial in minimizing mud filtrate invasion. Numerous studies document the effect of drilling mud components on rock wettability. Considerable effort is devoted to the measurement of relative permeability, both water-oil and gas-oil, in the laboratories of most major permeability, both water-oil and gas-oil, in the laboratories of most major oil companies and by commercial laboratories. These data are used in numerical simulations of field performance under various depletion scenarios. These performance predictions are used to make multimillion dollar investment decisions. Most laboratories attempt to preserve in-situ wettability by use of special coring fluids and by preserving the core at the wellsite to minimize further changes in wettability. Some laboratories go to considerable effort to make all water-oil relative permeability measurements at reservoir conditions on core having, as nearly as possible, the in-situ wettability. The reliability of relative permeabilities measured on so called ‘native wettability’ core samples must always be qualified because mud filtrate invasion usually modifies the water saturation and its distribution and may modify wettability. In mixed wettability rock (as defined by Salathiel), there is reason to question whether the original connate water saturation and distribution can be restored so that a primary imbibition water-oil relative permeability can be obtained. These uncertainties, in part, have caused a great diversity of opinion within the oil industry on how to measure water-oil relative permeability. Considerable improvement in data reliability could be achieved if coring mud filtrate invasion is eliminated as an issue. Invasion when coring occurs by three mechanisms as shown in Figure 1. First, core invasion may occur ahead of the bit. A filtrate bank builds up at low coring rates. Analysis of laboratory data in this paper indicates the interplay of filtrate interstitial velocity in the rock and core bit velocity through the rock control filtrate bank formation. Second, core invasion occurs at the core bit. Filtrate is generated at high rates due to bit cutting action.
Summary The effects of bit hydraulics while drilling shale with a standard three-cone bit are examined in this paper. Tests were conducted by drilling into large-diameter, intact shale samples at simulated downhole conditions in Drilling Research Laboratory's wellbore simulator. The shale samples were recovered from massive surface outcroppings and preserved for laboratory use. The effects of hydraulic horsepower from 20 to 400 hhp and bit weights from 20,000 to 50,000 lbm on rate of penetration are presented. Introduction Most deep shales and some intermediate shales found in the U.S. are typically "slow" drilling formations. Efforts to improve rate of penetration in shale have been performed in both field tests and small-scale laboratory investigations. Until recently, laboratory drilling studies on shale have been confined to microbit1 or single-cutter2 studies because the ability to obtain and preserve large, intact shale samples had not been developed. In addition, laboratory facilities where full-scale drilling tests could be conducted at simulated deep-well conditions did not exist. Massive surface shale formations have been located and techniques have been developed to extract and preserve large-diameter, intact samples. With these samples and the ability to simulate full-scale drilling conditions, a systematic, technical approach was taken where the effects of drill bit hydraulics were examined while drilling shale at simulated downhole conditions. Background and Definitions The in-situ conditions of a typical deep wellbore and surrounding rock formation are illustrated in Fig. 1. The formation is subjected to overburden stress, confining stress, wellbore pressure, and formation pressure. As previously demonstrated,3–5 rate of penetration is influenced strongly by the bottomhole conditions and appears to be most sensitive to the differential pressure between the wellbore and formation. These effects, however, can vary widely depending on rock properties such as rock type, strength, density, permeability, and mud properties such as composition, filtration rate, viscosity, solids content, and particle size.6 Bit hydraulics - i.e., the means of removing cuttings from the hole bottom and cleaning the bit with the drilling fluid - are a key factor in improved bit performance. Bottomhole cleaning theories,7 microbit studies,8 and full-scale laboratory drilling experiments in hard, impermeable rock9 have shown the need for adequate bit hydraulics to maximize rate of penetration and avoid bit balling. Field tests with extended nozzles10 also have demonstrated great potential for increasing rate of penetration in certain formations with improved bit hydraulics. Some nondrilling laboratory studies have shown the effects of nozzle size on pressure distribution at the hole bottom.11
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