Planning Workovers in Wells With Fault-Damaged Casing-South Pass Block 27 Field
Authors: McCauley T.V. 
Published: December 2007

Here is a statistical analysis of fault damage based on 165 workovers through wells cut by active faults. Damage was found in wells cemented through the faults and was correlated with the age of the well, depth of intersection, type of well (single or dual), year of workover, and years on gas lift. Also discussed are effective workover techniques and means of preventing damage to new wells.

Introduction

Casing damage resulting from slippage along active fault planes is a serious problem in workover operations in the South Pass Block 27 field at the mouth of the Mississippi River. Approximately 250 producing wells in this field intersect one of four producing wells in this field intersect one of four major field faults in a depth range where damage has been noted. Through Dec. 1, 1971, damage had been encountered in 54 of 165 workovers on fault-cut wells, resulting in the forced abandonment of 21 boreholes.Fault movement produced a casing centerline shift over a fairly short interval (less than 10 ft). Damage was normally not evident until a major workover was under way and full-gauge tools could not be worked through the casing opposite the fault zone. Swaging or milling was then required before workover operations could proceed. In cases where damage was severe, it was sometimes impossible to recover the tubing string opposite the fault, or to successfully mill through the damage without milling outside the casing. When this occurred, it was impossible to regain access to the wellbore below the fault, and the well was lost.The two primary goals of this study were:to develop a method of predicting damage in any fault-cut well before starting workover operations, andto determine a method of combating fault movement in both new wells and existing wells.

Realization of these goals enabled implementation of a more effective workover program, and minimized economic loss from continued fault movement.

Evaluation of Potential Damage To Fault-Cut Wells

All four of the faults that caused the damage were normal contemporaneous growth faults. Faults 1 and 3 have growth indices (ratio of downthrown section thickness to upthrown section thickness) from 1.1 to 1.3 throughout the depth range where damage had been noted. Fault 2 has a growth index ranging from 1.3 at 4,000 ft to more than 1.6 at 8,000 ft, and the growth index for Fault 34 ranges from 1.7 above 2,000 ft to more than 2.0 at 6,500 ft. Faults 1 and 2 are buried at approximately 3,000 ft. Fault 34 is active to the surface; mud slides have been noted along the marine outcrop of this fault.Faults 2 and 34, with higher growth indices, exhibited the most severe damage; 47 of the damaged wells and all 21 forced abandonments resulted from these two faults. Movement along Faults 1 and 3 appeared less severe, resulting in only 7 damaged wells in 44 workovers. In view of this fact, Fault 2 and Fault 34 were investigated separately. Faults 1 and 3 were not analyzed in detail.As a preliminary step, damage trends of these faults were plotted vs variables of the original mechanical condition of the fault-cut wells in question to determine what factors could be correlated with occurrence and severity of damage. The interrelationship of these factors was then explored with a statistical analysis computer program. On the basis of this, the probability of encountering damage or losing the well probability of encountering damage or losing the well was estimated for wells cut by these faults.

JPT

P. 739