Borehole Shape Characterization with Azimuthal LWD Measurements: Evaluation, Applications, and Limitations

Abstract

Abstract Caliper is a fundamental measurement of wellbores. It has significant impacts on decision making regarding well completion and monitoring operations. Traditional Calipers are measured mechanically, thus cannot be performed while drilling since they will not be able to survive the harsh conditions of drilling while the bottom hole assembly (BHA) rotates. With logging while drilling (LWD) measurements, several methods have been developed over the years to derive borehole calipers or hole shape indicators from LWD logs. In this paper, we will compare the LWD derived calipers with that obtained from wireline (WL) and summarize the developments, utilizations, and limitations of LWD derived calipers. Factors affecting LWD calipers will also be discussed to help operation petrophysicists for better log data quality control. Examples will also be shown for using LWD calipers to identify borehole washout, elipticity, breakout, and spiral-hole conditions. Knowing these borehole conditions in real time can help to make action plans to improve drilling practices and reduce drilling risks. After drilling, LWD calipers are used to correct LWD logs for borehole size effects and guide well completions. Introduction Logging While Drilling (LWD) derived calipers (ultrasonic, density, neutron, and resistivity) have been available for several years (Maeso and Tribe, 2001), but their utilizations are still limited to LWD log data quality control due to the belief that they are not as reliable as the mechanical calipers obtained from wireline (WL) logs. In fact, LWD calipers have their advantages over the WL calipers. One example is that they cover the whole circumference of the wellbore, thus provide a better characterization of borehole shape. A 3-D caliper image can be obtained using 16 radii density measurements around the wellbore. Major limitations of LWD calipers are that they are derived from LWD logs such as the density data. Therefore, they are valid only if the raw data quality is good. In situations where borehole washout is larger than a few inches, the density data will be compromised. Consequently, the density derived calipers will not be accurate. WL formation imaging tools provide a pair of mechanical calipers that are often used for washout and borehole shape analysis. However, these calipers are limited to two orthogonal directions. This study will concentrate on ultrasonic and density calipers and evaluate their applications and limitations. The focus will be placed on both drilling and geo-stress characterization since both will have significant effects on borehole size/shape. Real-time borehole size/shape analyses have the potential to allow a driller to optimize the drilling practice to improve borehole quality and drilling efficiency. Borehole information while drilling allows engineers to make better decisions regarding well completion and cement volume computation. The main objectives of this paper are to better understand the causes of hole-size variations for horizontal wells and investigate how these variations may relate to drilling process and/or rock mechanics. Attempts were also made to better understand effects of borehole shape on petrophysical measurements and methods to remove these environment effects.