Abstract

Final production rate alone is not an adequate measure of the success of a well completion. Rather, we must estimate the "potential" of a reservoir and judge the ultimate success of a completion on how close we come to achieving this potential. Specific productivity indexes (SPI's - BFPD/(PSI*FT)), specific injectivity indexes SII's - (BFPD/(PSI*FT)), and completion efficiencies (CE's - percent of Darcy radial flow) can be calculated at various times throughout a well completion. Analysis of these data quantifies the efficiency of the completion after each individual completion operation, allowing a determination of the effects of each completion practice to be made. In addition to completion efficiency data, a comparison of gravel placement volumes behind casing helps quantify optimum gravel packing procedures.

Twenty-two Gulf of Mexico completions have been analyzed using this technique. This paper will detail the results of this analysis, in particular the productivity effects of various methods of underbalanced perforating, gravel packing, and well control. Items of discussion include: the effects of underbalanced perforating on well performance, the effects of flowback after perforating on perforation tunnel cleaning, productivity impacts of various types of well control methods following perforating and gravel packing, and comparisons of gravel pack design parameters and gravel placement behind casing.

Introduction

A gravel packed completion consists of a series of connected events and procedures. These are:

  1. perforating and cleanup,

  2. running the gravel pack assembly,

  3. acidizing and/or pre-packing,

  4. annular gravel packing,

  5. running completion tubulars and installing the well head, and

  6. final flow to clean-up.

Randomly interspersed with these procedures are well control events such as spotting LCM pills and various well test operations including pressure build-up tests after perforating and flow or injectivity tests prior to pre-packing or gravel packing.

The final gravel packed well's CE depends upon this chain of completion events. Any one of these events may dominate the final well performance. Flow tests and injectivity tests are measures of the well's flow capability into (injection test) or out of (production test) the formation. Fluid loss rates are also measures of the well's injection capacity. Data obtained during injectivity tests can be used to provide a quantitative measure of the wellbore condition, which can be translated into either a completion efficiency or skin factor. The evaluation of these sequential data can be used to provide a time chart of completion effectiveness which can highlight sudden changes in the condition of the reservoir and pinpoint a specific operation that either enhances or damages the completion.

Objectives of the Study. As a follow up to the 1994 McLeod, Minarovic gravel packed gas well completion study, we analyzed 22 Gulf of Mexico gravel packed oil wells in an attempt to identify best completion practices. Well completion techniques within the Gulf of Mexico Division have varied greatly over the past several years. Often post audits have not been performed or have not been documented comparing the productivity of different completion methods, making evaluation of current completion practices difficult. Without a complete understanding of how current completion practices effect well productivity, improvements in completion technology can not be measured, making it difficult to achieve continuous improvement. To address these issues, the following objectives were established for this study:

  1. Quantify the "baseline" productivity values for current gravel packed oil well completions in the Gulf of Mexico, i.e., "Where are we now?"

  2. Identify and recommend completion practices which result in higher efficiency gravel packed completions, i.e., "How can we improve?"

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