Theory and Implementation of a Borehole Caliper Measurement Made While Drilling

Abstract

Summary Like comparable wireline measurements, measurement-while-drilling (MWD)formation-evaluation tools are dependent on the borehole condition. Because ofthis dependency, the borehole size must be known for appropriate boreholecorrections to be made to porosity measurements. This paper describes calipermeasurements that can compute the effective diameter of a uniform or nonuniformborehole with nonmechanical means. The basis for the caliper measurement is agamma/gamma density sensor that uses asymmetric detector placement to definethe borehole size. The derivation of the caliper is presented with field logcomparisons. Introduction Because sensor capabilities are being added regularly to MWD systems, bottomhole-assembly (BHA) design must now contend with variables absent in thepre-MWD era. New considerations include MWD assemblies of a given OD that areless stiff than the BHA components they replace I and, depending on theparticular drilling application desired, a different MWD sensor placed near thebit. Proximity of a given sensor to the bit is compromised further by Proximityof a given sensor to the bit is compromised further by the increasing use ofsteerable systems. These, by their very nature, require a motor near the bit, and directional sensors require additional nonmagnetic collars in proximity. All this requires that formation-evaluation-while-drilling (FEWD) sensors beplaced as far as 200 ft from the bit. In some drilling placed as far as 200 ftfrom the bit. In some drilling environments, this can result in the boreholebeing significantly enlarged by the time FEWD sensors evaluate the rock. Borehole enlargement is of particular concern for two reasons. First, from adrilling viewpoint, whether the hole enlargement occurs rapidly or slowly mustbe determined; i.e., is the borehole stable or unstable, and does knowing thestability of the borehole improve our understanding of the drilling conditions?Second, from a FEWD viewpoint, can we infer or measure the true borehole sizeso that the formation-evaluation sensors can be interpreted properly. This isvery important in the case of nuclear formation-evaluation tools because, likestandard wireline nuclear tools, they are very sensitive to boreholeconditions. This borehole sensitivity implies that, to achieveformation-evaluation quality comparable with wireline, comparable knowledge ofthe borehole while drilling is necessary. Borehole enlargement now can bemeasured while drilling with a nonmechanical sensor that uses multiple detectormeasurements to determine the average borehole caliber. This average can thenbe displayed as a continuous curve in the same manner as traditional mechanicalcalipers. We calibrated the new borehole caliper measurements for various muddensities and borehole diameters. We observed that repeat sections or wipedlogs after drilling indicate the same size or larger borehole. We obtained goodagreement (within 3% to 10%) between dipmeter (x-y) caliper measurements andthe measurements made while drilling and found that the geometric average ofthe wireline x-y dipmeter caliper was larger than or equal to the measurementmade while drilling. These results were obtained at drilling rates ofpenetration (ROP's) of up to 500 ft/hr. A borehole-size measurement made whiledrilling allows us to make the borehole corrections required by nuclearporosity sensors and to calculate the cement volume without making a separatetrip into the borehole. Design Constraints MWD assemblies are used in the fall range of well types, from straight-holeexploratory wells to horizontal wells. Consequently, these assemblies must beincorporated into the full gamut of BHA types. Nonmotor BHA designs arecategorized as building, holding, and dropping assemblies. The relativeplacement of only three full-gauge stabilizers near the bit produces the fullrange of desired angle changes, from aggressive build to aggressive drop. Smallextents of undergauge on the blades (less than 0.5 in.) moderate aggressiveassemblies. The foregoing emphasizes that the placement and diameter ofstabilizers are key to controlling build/drop tendencies. To remain minimallyintrusive to this process, we specified that our porosity sensor (andassociated caliper device) design not require wall contact. This specificationhas the virtue of not complicating drilling situations where slick ornear-slick assemblies are dictated by hole conditions. A final advantage is inpendulum dropping assemblies, where the closest available pendulum droppingassemblies, where the closest available stabilizer is commonly 60 ft from thebit. In addition, because the borehole is rarely circular, the caliper must becapable of making valid measurements in oval or elliptical boreholes. Finally, because the nuclear FEWD sensors are sensitive to the borehole size andenvironment (such as mud density), an MWD caliper must have an accuracy on theorder of 0. 125 in. We imposed three restrictions, which are inherent to our MWD borehole caliper measurement:there can be no mechanical protuberancesto interfere with the drilling operation,the protuberances to interferewith the drilling operation,the caliper measurement must be capable ofaccurately averaging a noncircular borehole, andthe caliper must beaccurate to 0. 125 in. Theory of Operation To meet these restrictions, we used the Simultaneous Formation Density (SFDSM) sensor to make the required measurements. The SFD sensor is the samegamma/gamma density too] described by Paske et al. 5 Fig. 1 shows the tool incross section. The basic tool has four detector banks on the circumference ofan insert placed inside a drill collar. The detectors are placed with two axialspacings to the cesium- 137 gamma source. The front detector bank in Fig. 1 isdirectly above the cesium source, and axially, is farthest from the source. Theother three detector banks are placed at the same axial distance from thecesium source. By placing the SFD tool in the various test pits shown in Fig.2, the formation response of the individual detector banks can be determined asfunctions of the borehole diameter and the borehole mud density. These testpits consist of four limestone rock stacks and 12 density tanks. The tanks areused to determine the mud density effects on the nuclear sensors. Fig. 3 showsthe borehole dependence for one detector bank. The solid line fit in Fig. 3 isgenerated from the equations below.