Acquisition of Directional Survey Data with Adapted Logging Tools
In the PEMEX Southern Drilling and Producing Region, which includes all land wells drilled in the southeastern states of Mexico, directional drilling has gradually increased to 75% of wells drilled. Producing zones in the region are also deep with difficult targets under high pressure and temperature. These factors have led PEMEX to seek out new technology in an effort to optimize drilling and producing operations.
One of the special projects defined for the year 2001 was to evaluate the application of running a high-accuracy gyroscopic directional survey instrument in tandem with PEMEX logging systems. In the process, the Southern Region engineering group and the PEMEX wireline logging division worked with a local survey company to build a special housing to run the gyro probe on top of several different logging instruments.
This paper describes the development of the tandem gyro-logging combination tool and tests in three wells. System performance is analyzed and comparisons are made between gyro survey results and other survey instrumentation. Conclusions indicate that the success of running two services in one run in the well saves substantial rigtime and eliminates the costs of additional wireline runs. Furthermore, the application of high-accuracy survey information may prove valuable in more accurate wellbore placement and reservoir mapping—leading to enhanced production. In the future, the combination run may also have value in detecting the orientation of imaging logs in both cased and open hole to determine fracture orientation and formation dip angle.
In early 2001, the engineering group in the PEMEX Southern Region Drilling and Producing Division began discussions with a local directional survey company concerning a new wellbore logging application that combines a high-accuracy directional survey instrument with downhole logging systems. After careful consideration, the PEMEX group made the decision to pursue the application as one of its year-2001 special projects to evaluate and deploy new technology in an effort to reduce costs and add value to PEMEX operations.
The PEMEX Southern Drilling Region comprises the land area extending from Agua Dulce, Veracruz on the western boundary to the Yucatan Peninsula in the east. The region’s oil provinces, such as in the Chiapas and Tabasco state areas, contain challenging geological stratigraphic traps under high pressure and temperature. Wells typically are drilled to 4,500 to 5,000 meters with bottomhole temperatures often exceeding 150 degrees centigrade and mud weights greater than 2 gr/cc.
Due to reservoir structure, terrain and environmental concerns, in recent years directional drilling in the region has increased to 75% of wells drilled-increasing the need for more accurate methods of wellbore surveying and placement. It is well documented that rate-gyroscopic survey systems, adapted from aerospace guidance technology and introduced to the oil industry in the 1980’s, provide a higher level of survey accuracy than magnetic systems or older gyro techniques.1,2,3,4
In general, rate-gyroscopic survey systems provide a level of quantifiable survey accuracy of 0.1 to 0.3 percent of hole depth laterally, compared to 1 to 3 percent with magnetics and older gyros. In reality, survey performance is more complex, varying by such factors as region, method, age/condition of instrumentation, tool alignment, and wellbore inclination angle and direction. From a geological standpoint, accurately measuring azimuth and inclination with a survey instrument can be crucial to precise wellbore placement and reservoir mapping, but also of great concern is the method and correlation of depth measurement.