Evaluating And Planning Directional Wells Utilizing Post Analysis Techniques And A Three Dimensional Bottom Hole Assembly Program
Authors: Millheim Keith Karl , Gubler Ferdinand H. , Zaremba Herb B. 
Published: December 2007

Detailed post-analysis can significantly reduce the cost of drilling complex and troublesome wells. It can lessen the learning curve for drilling in a particular area and assist in the training of inexperienced drilling personnel.

Over the past three years, 72 directional wells have been post-analyzed in detail. Sixteen of these have been in the North Sea. From this work, a simple technique has evolved to post-analyze a well in such a manner that the complex interactions of the key variables are understood. These include: weight-on-bit, rotary speed, penetration rate, hole inclination and direction, geology, bits, and bottom hole assemblies. Post-analysis can be used to assess whether the trajectory changes were man induced or caused by the geology. This type of post-analysis does not require the use of a computer program.

A computer program is necessary to analyze the complex behavior of the bottom hole assembly. A finite element algorithm was developed to do this. The program was designed to calculate the three-dimensional forces and displacements associated with any bottom hole assembly subjected to axial loading in a borehole. The borehole can either have a constant or variable inclination and direction. The program can be used to analyze bottom hole assemblies of wells that have been drilled (post-analysis) and/or help plan bottom hole assemblies in future wells.

Further applications of the post-analysis technique is the evaluation of a trajectory change using a bent sub with a mud motor or by jetting with the pumps. A new useful method of evaluating the ideal performance of a mud motor correction run is presented with examples.

A complete development of the post-analysis technique is presented. An interpretation of a typical directional well that has been post-analyzed is cited. Application of the computer program to evaluate some of the assemblies that were run in the example is covered. A new technique to analyze and plan deflection tool runs for any inclination angle is presented with examples.

Introduction

In 1972, a research project to optimize directional wells was initiated which required detailed drilling information. The normal drilling data was available from drilling recorder charts, bit and mud records, and tour sheets. However, detailed information on bottom hole assembly (BHA) configurations, dimensions and where the assemblies were run was virtually non-existant or not usable. Detailed data of weight-on-bit and rotary speed was also scarce. Forms specially designed for directional wells were drafted and sent out to the field for completion. Response was good. Within two years, detailed drilling data on more than 50 wells had been accumulated. Data was obtained from the Gulf of Mexico, Wyoming, Texas, Canada, Michigan, the North Sea, Trinidad, Iran and Holland.

By 1974, the effort to understand the mechanism of directional drilling had not significantly progressed. Between 1972 and 1974 an approach was taken to physically observe the directional drilling operation and learn by experience, like a directional driller. The outcome was that the observer became an excellent directional driller, but a full understanding of the total mechanism of the variable's interaction was not learned.

One of the major difficulties in interpreting trajectory behavior was the lack of being able to accurately analyze the bottom hole assembly behavior. To alleviate this deficiency, a computer program was developed that would calculate the three-dimensional displacements and forces for any type of bottom hole assembly. To do this a finite element algorithm was used.1

The first round of post-analysis using the new program was very disappointing. Results in most part did not agree and in some cases were completely out of phase. However, it was this work that led to recognition of the importance of hole curvature on the bottom hole assembly response.2