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Abstract

Data obtainable with a low cost wellsite total gas detector can provide valuable assistance in production drilling. Gas liberated by the drill bit production drilling. Gas liberated by the drill bit into the drilling mud is an effective tool for accurate perforation selection even if wireline log interpretation proves inconclusive. Gas produced into the mud system by the formation provides valuable assistance in detecting and monitoring abnormal pressure. Optimum mud weight can also be determined pressure. Optimum mud weight can also be determined from produced gas data.

Introduction

In today's world of unstable oil prices and inflated drilling costs, wellsite services are often the casualty. In most cases, development wells do not qualify for exploration type services in the best of times. For these reasons computer based and conventional manned hydrocarbon well logging services (mud logging) are often considered not to be viable in production drilling. This does not mean, however, that vital gas detection information is unavailable.

The basic purpose for drilling is to locate and produce hydrocarbons. Success in this endeavor can produce hydrocarbons. Success in this endeavor can be enhanced with "Exploration type" data at relatively low cost. The careful and detailed evaluation of data obtained from a relatively inexpensive well-site total gas detector can aid in the location of hydrocarbons and can assist in the exact placement of production perforations. As drilling proceeds, wellsite gas detection data can provide valuable assistance in detecting, monitoring and controlling abnormal formation pressure.

PERFORATION SELECTION PERFORATION SELECTION
Basic Principles of Liberated Gas

"Liberated Gas" present in drilling mud has been defined as, "Gas mechanically liberated by the bit into the drilling fluid as the bit penetrates the formation". Figure one illustrates the concept of liberated gas. As the bit penetrates the bore hole it mechanically crushes the formation and introduces the contained hydrocarbons to the circulating mud system. A penetration rate curve often indicates the thickness of the hydrocarbon liberating interval because the porous hydrocarbon containing rock usually drills faster than the non-porous rock above and below. One lag time after the porous rock is penetrated, the gas detector will indicate an enrichment of hydrocarbons in the drilling mud. This enrichment is described as, "Liberated Gas". It is important to note that only liberated gas makes up the increased mud gas concentration when the mud column pressure is greater than the formation pressure.

Although the drilling mud may contain other categories of gas, such as, "Produced", "Recycled" or, "Contamination", the principle function of the wellsite gas detector is to identify and locate with respect to depth all instances of liberated gas. Unless the liberated gas is accurately correlated to its source, such gas is of little value in accurate perforation selection. Fortunately, changes in penetration rate do not normally destroy evidence of the boundaries of liberated gas zones. Increased penetration rate tends to increase the concentration of gas in the drilling mud, but clear indications of the beginning and ending points of liberated gas are usually preserved. A review of actual gas detector instrument charts may be required if the plotted data does not provide sufficient detail.

If the boundaries of liberated gas zones can be accurately defined, then it follows that liberated gas data can serve as a qualifier of porous intervals observed on wireline logs. Synergistic use of porosity curves and total gas curves is enhanced if both sets of data are plotted on a common grid system. The Society of Professional Well Log Analysts has introduced recommended practices to the Well Logging Industry which will enhance this benefit. Fortunately, liberated gas is not subject to errors of measurement occasionally present in wireline log curves.

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