Bore Hole Failure Near Salt Domes
Authors: Bradley William B. 
Published: December 2007

A model of bore hole failure has been used to analyze the drilling problems which occurred on the "C" platform in the Eugene Island, block 175 field. Two wells have been analyzed and an explanation has been developed which shows why problems occurred in one well, but not in the second. The results show why drilling near salt domes can produce significantly increased bore hole failure problems.

Introduction

Bore hole stability is a continuing problem which results in substantial yearly expenditures by the petroleum industry. As a result, a major concern of the drilling engineer is keeping the bore hole wall from falling in or breaking down. Detailed attention is paid to drilling fluid programs, casing programs, and operating procedures programs, casing programs, and operating procedures in drilling a well to minimize these costly problems. problems. One of the difficulties the drilling engineer has to cope with are the many variables which influence the stability of bore holes. This paper discusses how the bore hole failure theory discussed in reference 1 can be used to assist the drilling engineer in understanding bore hole failure and in designing drilling programs to minimize these problems. problems. This paper discusses the application of this failure model to the problem of drilling near salt domes. The bore hole failure model has been used to explain the presence or lack of bore hole stability problems in two wells drilled from the "C" platform in the Eugene Island, block 175 field. platform in the Eugene Island, block 175 field. From this analysis, the development of mud weight operating curves is illustrated.

BORE HOLE FAILURE NEAR SALT DOMES

The stability of bore holes is greatly affected by the conditions of stress in the earth (in situ stresses). The greater the mean in situ stress and the greater the in situ stress difference (sigma - sigma), the greater the problem with bore hole instability. The presence of a salt dome can significantly alter the state of in situ stress from the normally stressed condition and as a result, the stability of bore holes in close proximity to salt domes can be greatly affected.

Little work has been done on the prediction of in situ stresses in the formations around salt domes. However, some unpublished work indicates that important changes in the in situ stress state can occur. Indications are that regions near tie shoulder and the flanks of the salt dome can be significantly affected. In formations adjacent to the flanks of the salt dome the in situ stress in the hoop direction around the salt dome may be lowered significantly below the normally stressed value which occurs at some distance from the dome.

The reduced in situ hoop stress resulting from the presence of a salt dome can significantly increase both lost circulation and bore hole collapse problems. For example, initial wells drilled at some distance away from the salt dome may not experience either significant lost circulation or bore hole collapse problems. Whereas later wells drilled using the same drilling parameters may experience increased problems as the wells are drilled in closer and closer proximity to the salt dome. An example of the influence of salt domes on bore hole failure is discussed in the next section.

AN ANALYSIS OF THE DRILLING PROBLEMS ON THE EUGENE ISLAND, BLOCK 175 C-PLATFORM

C-13 Well Description:

The C platform and the C-13 well are located with respect to the salt dome as shown in Figure 1. The C platform is located on the north side of the salt dome in the Block 158 OCS G-1220 lease. Figure 2a shows a cross section through the dome in the plane of the C-13 well. The well profile is for the C-13 sidetrack No. 2. A caliper profile is for the C-13 sidetrack No. 2. A caliper log and the lithology for the C-13 sidetrack No. 2 in the depth interval of interest are shown in Figure 3.