The Principles and Procedures of Geosteering

Abstract

Abstract The management of directionally drilled wells has recently progressed to the stage where targets have been reduced in size to a point in the earth with no tolerances. These targets can and are changed during the drilling process. The management of these point moving targets, usually in high angle or horizontal wells, is a precise form of directional drilling now called geosteering. It is this precise placement of the wellbore that creates the value in drilling these wells. Several technologies have made this advancement possible. These include reliable steerable systems, improved and new physical formation measurements, log data modeling, sensors near the bit and instrumented motors, and detailed reservoir mapping with the help of 3D seismic processing. Most papers on geosteering have concentrated on one of these advances. This paper addresses how these technologies are merged to execute the successful geosteering project. New systems, sensors, and computations have created a mass of data and control parameters that require real-time decisions by operator personnel based on technical recommendations from service companies. These decisions have a critical impact on the net worth of a project. The team involved must rely on each other's expertise and understand the overall objective of a geosteering project. To do this, it is paramount that data from these different technological areas be merged into a 3D visualization encompassing geological structures and drilling trajectories. Patterns in procedures have developed from these projects over the past three years which demonstrate the difference between a successful project and a failure. The examples in this paper illustrate the techniques that will enable a change in the outlook, towards precision directional drilling and completion projects. Introduction Horizontal drilling is used to remedy a wide range of production problems in developed or developing oil and gas fields. These problems include water and gas coning, disappointing production due to tight formations or difficult completions, and poor return due to isolated producing zones. The horizontal remedy in these cases depends on an accurate placement of the horizontal segment in the objective formation. The value of such a project critically depends on this placement. The tolerances for landing this horizontal well are small and require an accuracy beyond what is currently available in survey sensors. The target must be tied to geological information gained during the drilling process. This is geosteering. The concept has been written about previously. A good definition is the planned interactive placement of a wellbore using geological criteria. It is similarly defmed in other works. An early horizontal well drilled in 1989 is an example of geosteering as shown in Figure 1. The objective formation was a series of sands of which the two lower sands were the primary targets. The sands were located in a gas/oil/water interval. A pilot well was drilled to verify the sand locations. It found the sands 30 ft lower than expected, and from the operator's point of view, too close to the water contact for conventional targeting. Drilling the horizontal section was put on hold. The project was revived when it was suggested to geosteer a wellbore initially following the N2L sand. Seismic data indicated that two small faults (with throw less than 25 feet) could be expected. A trajectory was designed to account for this, which is shown as the "mod plan" in the figure. The well was landed in the sand as expected. Drilling continued up dip at 91.5 degrees inclination. Gamma ray data was being used to geosteer. At point "a", it indicated that the sand was shaling out. Trajectory data indicated that the wellbore was out of the bottom of the sand. A deviation from the plan up into the cleaner part of the sand was recommended and after discussion accepted. This was a major event for directional drilling in 1989. The maneuver worked, and the sand was reacquired. The well path was brought back to 900 in anticipation of the first fault (point "b"). The fault was crossed and the well drilled into the upthrown N3 sand which was very clean and oil filled (point "c"). P. 133