A Proven Gas-Well Completion Technique for Higher Deliverability
Authors: White Bill , Walker Terry , Diebold Joe 
Published: December 2007

An investigation was initiated into the mechanics of gas-well perforating because of a number of cases of drastic increases in productivity of gas wells after reperforating the same interval. Since these reperforations were with gas at low pressure in the wellbore, a test series was set up in a flow laboratory to investigate the factors involved. Berea sandstone was perforated with gas, kerosene as a formation fluid. Tests were made over a wide range of pressure differentials both to the formation and in the wellbore. The results of the test series show that the fluid pressure conditions in the wellbore at the time of perforating are much more important for gas-filled than for liquid-filled formations. For liquid-filled formations no plugging occurred in the cores tested that could not be removed by fluid flow from the core. A practical completion technique has been developed and put into practice to control the environmental conditions while perforating gas wells.

Introduction

Many gas wells do not come up to expectations of deliverability based upon open-hole drill-stem tests or by comparison with other wells completed in the same reservoir. This discrepancy between expectations and actual deliverability is more common with gas wells than with oil wells. High damage ratios, indicating severe skin damage, are also quite commonly observed in gas wells which have been completed with the "normal" completion methods, where wellbore pressure is maintained above reservoir pressure at the time of perforating. In many states, gas-well daily allowable is based on absolute open-flow potential. With the rapidly increasing demand for gas at more attractive prices and a ready market. it becomes much more important to obtain the maximum potential and deliverability from each well completed in a reservoir.

Evidence of Poor Completions in Gas Wells

Gas wells, which in comparison to nearby wells had not potentialed at a satisfactory rate, have often been reperforated. High-pressure control equipment was placed upon the well-head and entry made against pressure with a through-tubing gun. The new perforations were usually in the same interval as the original perforations, and the results were vastly improved potential tests for the well. The consistent success in improving deliverability of these gas wells indicates that the "normal" completion technique produced results which were inferior to the technique used in reperforation. A factor in the reduced deliverability of gas wells is apparent in the problems associated with blast joints. Numerous observers in the field and in the technical literature have repeatedly pointed out that where a blast joint is employed, seldom more than one to four shiny spots or holes are ever seen on the blast joint, regardless of the number of perforations in the casing. This indicates, of course, that from one to four holes produce the entire amount of gas coming from this upper completion. Presumably the normal completion technique left the remainder of the perforations ineffective. Several laboratory investigators have simulated the normal oilwell completion technique of wellbore pressure higher than formation pressure and an extended time of filtration into the formation after perforation. The tests showed that the flow efficiency of the perforations was reduced under these conditions. These results led to considerable improvement in the flow efficiency of oilwell completions. However, all of the reported tests have been made with a liquid in the test sample. In view of this. and the problems on gas-well completions, a series of tests on gas-saturated samples was started to determine if a significant difference exists in the results of perforating gas-bearing formations as compared to oil-bearing formations.

Laboratory Procedures and ResultsThe objectives of these tests were to determine the relative effects of pressure conditions and fluid conditions in the wellbore at the time of perforation of gas-saturated samples in comparison to the previous work done on oil-saturated samples.

JPT

P. 647ˆ