Borehole Measurements of the Complex-Resistivity Dispersion Spectrum: A New Logging Method to Identify Low-Resistivity Reservoirs

Jiang, Ming (China University of Petroleum) | Ke, Shizhen (China University of Petroleum) | Kang, Zhengming (China University of Petroleum) | Sun, Xu (China University of Petroleum) | Yin, Chengfang (China University of Petroleum)

OnePetro 

ABSTRACT

Identifying low-resistivity reservoirs, which are important oil reservoirs with considerable productivity, is a real challenge to conventional electrical resistivity logging methods. For this reason, this paper introduces a new spectrum logging method for effective identification of low-resistivity reservoirs. This method measures the complex-resistivity spectrum of formation, which is sensitive to water-filled porosity without the influence of low resistivity. This paper presents the relationships between the complex-resistivity spectrum and petrophysical properties based on laboratory data. The electrode array of the prototype for borehole testing is also described. A numerical simulation was performed to study the detection characteristics of this prototype and field tests were conducted to verify the feasibility of this method. The results show that variations in water-filled porosity obtained by this method show a good fit with the actual values. Further, water saturation can be obtained for oil reservoirs, especially for low-resistivity reservoirs. This logging method provides an approach to characterize oil reservoirs that is more effective than existing methods for the case of low-resistivity reservoirs.

INTRODUCTION

During the past four decades, an increasing number of low-resistivity reservoirs have been discovered in new locations. Today there are many documented records of low-resistivity reservoirs in locations including the Gulf of Mexico, North Sea, Indonesia, Venezuela, and the Tarim Basin. Several studies have proven the productivity of these reservoirs (Zemanek, 1989; Souvick, 2003), but the term ‘low-resistivity reservoir’ is rather relative and lacks an absolute description. Low-resistivity reservoirs not only imply low absolute values of resistivity, but also a lack of positive contrast in measured resistivity between the reservoir and the water zone, making them difficult to identify using traditional electrical logging data. There are many causes of low-resistivity reservoirs, but there is usually only one primary cause in any given location. For example, Gulf Coast (USA) and Misoa Sands (Venezuela) are caused by laminated shaly sand (Ruhovets, 1990; Coll et al., 1996), Midway Sunset Field (California, USA) and Potiguar Basin (NE Brazil) are caused by fresh formation waters (Wharton et al., 1981; Condessa, 1995), and the Simpson Series (Oklahoma, USA) is caused by conductive minerals (Schulze et al., 1985).