Environmental Parameters Affecting Neutron Porosity, Gamma Ray, and Resistivity Measurements Made While Drilling

Abstract

SPE Members Abstract Recent technological advances have resulted in development of formation logging sensors contained within drill collars. Environmental parameters affecting natural gamma ray, resistivity, and neutron porosity measurements while drilling (MWD) include borehole size, invasion, drilling rate, and formation exposure time. MWD logs, in many cases, can provide superior quality data as compared with comparable wireline sensors. Introduction Measurement while drilling is a relatively new, emerging technology. Only recently has the oil industry begun to accept the possibility that logging measurements made while drilling might provide formation data of similar quality to wireline. Making resistivity measurements while drilling, rather than on a wireline after removing the bit from the well, was an early concept limited by technology. As early as 1935, Karcher, et al, attempted to make resistivity measurements while drilling, however, their success was limited. Multiple attempts at measurements while drilling were conducted throughout the 1930's. Some of these attempts were marginally successful, but none were technologically feasible on a commercial level. J. J. Arps invented the concept of mud pulse telemetry in the 1950's, and there were many attempts at the use of this concept, and others, throughout the 1960's and 1970's. Once again, these concepts, although practical by design, were not feasible technologically. The first commercially reliable MWD was a directional sensor introduced in 1979. Soon after, in 1980, the first successful MWD Gamma Ray (GR) and 16" short normal resistivity tools were introduced. This pioneering work was continued in 1984 with the commercial introduction of the first induction type resistivity measurement. In 1986 and 1987, laterolog systems have also been introduced. Companies are known to be testing other types of resistivity sensors. Clearly the next step in the evolution of the measurement while drilling process includes the development of porosity sensors. In 1986, a compensated neutron porosity sensor was introduced for commercial use. A density porosity sensor has been successfully field tested in 1987, and is expected to be commercial sometime this year. Early MKD logging tools were not very reliable, and required considerable attention to the drilling system hydraulics to achieve acceptable mud pulse detection. Measurements of gamma ray and resistivity made by the earliest MWD logging tools, although crude, provided useful geological correlation data. These tools were expensive to operate and were used mostly on high exposure drilling programs. Small savings in rig time or small improvements in early time decision making on these projects could achieve large dollar savings and/or increased efficiency. These early tools were clearly, however, not providing information of sufficient quality to be used for quantitative formation evaluation. As the technology progressed, pulse detection and tool reliability improved dramatically, and by 1984, the industry was providing excellent quality gamma ray and resistivity data. Additional early applications for MWD were to accurately pick coring points and critical casing points, and as "insurance logs" in difficult or high exposure wells in the event the well was lost prior to wireline logging. P. 155^