Advances in Openhole Well Logging

Abstract

Distinguished Author Series Summary Openhole well logging comprises a broad spectrum of measurements that includes mud, measurement-while-drilling (MWD), and conventional wireline logs. These measurements are the primary source of formation evaluation data used in applications ranging from individual drilling-well appraisals to comprehensive reservoir description studies. Openhole well-logging technology continues to evolve to address the needs for improved accuracy in measuring reservoir properties. Numerous technology advances have recently occurred. This paper presents several examples of new developments and applications to illustrate the current state of technology and to offer insight into future developmenttrends. Introduction Significant developments have recently been made in openhole well-logging technology, despite a major downsizing in the industry. New measurement tools and software systems have been introduced in an effort to provide accurate, cost-effective methods for addressing a variety of formation evaluation challenges, which include thin-bed evaluation, low-resistivity pay analysis, productivity/permeability prediction, horizontal well evaluation, fractured reservoir analysis, and reservoir geology characterization. Advances have been made in a variety of logging technologies. Enhancements in log data acquisition include higher data sampling and transmission rates and expanded computer capabilities to process log data and to control logging operations. The latest borehole imaging tools offer improved image quality from both microresistivity and acoustic imaging logs, which has led to growing applications. New multielectrode induction and laterologs provide enhanced thin-bed resolution and new formation measurements. Source, detector, and processing enhancements offer improved porosity and elemental analysis accuracy from nuclear logging tools. New acoustic logs provide multipole source and receiver arrays to measure compressional, shear, and Stoneley wave data in both hard and soft formations. In addition, new nuclear magnetic resonance logs offer greatly improved data quality for determining porosity, irreducible water saturation, and other formation and fluid properties. Wireline formation tester developments provide new downhole measurements, pump-out capabilities, and a modular design that allows multiple configurations. Advances in MWD logging include new resistivity measurements, improved porosity/lithology logs, and new applications. Finally, developments in log data processing and interpretation provide faster processing, more information from the log measurements, and more effective integration of data from multiple sources, such as drilling, mud logging, core analysis, geology, testing, and production. Data Acquisition The growing trend of increased numbers of downhole sensors and greater measurement sampling frequency for improved resolution has put new demands on well-log data acquisition systems. These systems comprise the hardware and software for sending data uphole from the logging tool; for receiving, processing, verifying, presenting, and storing data at the surface; and for electronically transmitting data from wellsite to office locations. Developments in downhole signal processing and data compression techniques are dramatically increasing the uphole data transmission rates to meet the requirements of new measurement tools. New wireline systems provide rates exceeding 500 kbit/sec, and future systems offering several megabits per second are expected in the near term. MWD transmission rates, based on mud-pulsetelemetry, have also shown improvement, but they remain a limiting factor for real-time data acquisition in some cases. MWD data transmission rates have recently increased from 3 to 12 bits/sec, and further improvements are foreseen. The capabilities of computer surface equipment to process log data and to control logging operations have also increased significantly. This has resulted in more efficient logging operations, improved quality control of log data, enhanced wellsite log analysis products, and improved methods for storing and transmitting data. These improvements have been made possible by using high-speed, multitasking workstations in wellsite logging units. Borehole Imaging Significant improvements have recently been made in the quality of borehole imaging measurements, which have led to growing applications of this technology. These measurements are commonly used for thin-bed evaluation, fracture identification and analysis, structural and stratigraphic dipanalysis, sedimentary facies analysis, and textural analysis. Technology developments have led to (1) better assessment of reserves in thinly bedded reservoirs; (2) improved definition of fracture density, aperture, and orientation for producibility estimation and perforation placement; and (3)greater geological information, such as definition of structural dip, paleocurrent direction, rock facies, flow barriers, and porosity types. Ref. 4gives a good overview of borehole imaging technology and its applications and contains a collection of technical papers describing the development of borehole imaging from its beginning until 1989. The primary techniques for acquiring borehole image data use electrical (microresistivity) or acoustic measurements. Microresistivity Imaging. Microresistivity imaging tools produce an image of the borehole wall by mapping its resistivity with an array of small, pad-mounted, button electrodes. P. 693^