Skin Calculation and Other Applications with New Sonic Measurement in the Gulf of Thailand

Abstract

Abstract Since the first sonic logging tool used for the petroleum industry in 1950s, we have seen many generations of acoustic measurements. The hardware and software changes made over the decades focused mainly on the improvements of compressional and shear measurements which are one dimensional in the near-wellbore region. To understand the complicated reservoirs today, there is a need to look at the compressional and shear slowness in three dimensions. The present wireline sonic logging tool that is equipped with a long receiver array and near and far monopole transmitters enables us to quantify the compressional slowness of near-wellbore damaged formation and the far-field undisturbed formation. This radial measurement adds an extra dimension to reservoir characterization. The compressional radial profiling clearly shows the formation damage across part of the reservoir. By combining it with the pressure drawdown mobility taken from undamaged and damaged zones, one can, for the first time, measure the mechanical skin just after drilling. The information is available at the wellsite. It helps to optimize the real-time data acquisition and completion selection. The fluid identification is one of the challenges in the deeply invaded reservoirs. The depth investigation of new sonic measurement is much deeper than conventional neutron density logs and previous sonic logs. Other applications including gas detection in deeply invaded reservoirs provided unique solutions in the Gulf of Thailand. The field example also demonstrates an enhancement in vertical resolution with the multiple-shot processing technique. This is much needed for evaluating the thin reservoirs. Introduction Traditionally, the total skin is calculated by well tests. The limitation is that the well tests do not measure the individual skin parameters. The methods for calculating the skins are well documented in many papers. Pucknell and Clifford (1991) discussed various methods. However, there are many assumptions, including the radius and permeability of the damaged zone. The mechanical skin caused by drilling and perforation is one of the most difficult skins to measure. While the modern perforation technique can reduce or eliminate the perforation skin, the drilling damage remains the same. In this paper, a new method for evaluating the drilling damage is introduced. It uses the new sonic data which provide direct measures of the formation damage (or alteration) radius. Methodology The principles and applications of the new 3D sonic measurements are well documented in Pistre et al. (2005). Compressional radial variations are determined by evaluating both the short- and long-spacing monopole measurements. Short spacings probe the formation in the near field (shallow), whereas longer spacings probe the formation in the far field (deep). This is graphically represented in Fig. 1.