Application of Drilling Performance Data to Overpressure Detection

Abstract The previously recognized effects of pressure on rate of penetration havebeen adapted to an overpressure detection technique for the Texas-LouisianaGulf Coast. It is postulated that, under specified conditions, a plot ofincremental rate of penetration vs depth will define an ever-decreasing trendin the normal pressure section and that this trend will reverse whenoverpressures are encountered, thus permitting the detection of overpressuresfrom drilling performance data. It also is postulated that a relationshipbetween rate of penetration and differential pressure exists. Relationshipsbetween pressure and rate of penetration as developed by several investigatorsare reviewed. Methods are developed to normalize rate of penetration data withrespect to some of the more significant drilling variables (bit weight, rotaryspeed and bit diameter). Using these methods, drilling performance data areanalyzed to determine if the postulated correlations mentioned can berecognized from actual field data, and results of these analyses arepresented. Introduction The presence of overpressured formations (formations with abnormally highfluid pressure) is a significant characteristic of the Texas-Louisiana GulfCoast geologic province. Dickinson originally outlined the occurrence ofoverpressured formations in this province, and both Dickinson and Hubbert andRubey presented theories on the origin of these high-pressure zones. Subsequently, drilling practices and mud and casing programs have beendeveloped which permit the detection and control of overpressures thusenhancing the chances of successfully drilling these zones. Many of thesetechniques, which are not in the literature, have been used extensively byvarious operators in the Gulf Coast for several years. Recently, log analysismethods have been reported by Hottman and Johnson using the transit time andresistivity of shales toidentify the first occurrence of overpressures, andestimate formation pressure gradients. This paper presents a techniquedeveloped for identifying the first occurrence of overpressured formations frominterpretation of drilling performance data. Specifically, rate of penetrationdata, by virtue of its dependence on differential pressure (the bottom-holepressure difference between the mud column and the formation), can be used toidentify overpressures. Data from this technique are immediately available as awell is drilled which is an obvious operational advantage over the log analysismethods reported earlier. Studies in the industry have shown that rate ofpenetration is considerably reduced by an increased mud column pressure. Assuming an inversely proportional relationship between rate of penetration anddifferential pressure, consider drilling (under constant conditions) a givenrock in the normal pressure section at ever-increasing depths. The totaldifferential pressure will increase with depth, and rate of penetration shoulddecrease. Consider drilling the same rock under the same conditions as theoverpressure section is entered. As the formation pressure gradient increases, the differential pressure deceases and the rate of penetration should improve. Thus, for constant rock properties and drilling conditions, it is postulatedthat a plot of incremental rate of penetration vs depth should define anever-decreasing trend in the normal pressure section, and that the trend shouldreverse when drilling into overpressures (Fig. 1). Such a plot would showcharacteristics similar to the shale resistivity and shale transit time plotscurrently used for overpressure detection, and would be a valuable supplementto these log analysis methods because it would be available as a well isdrilled. Furthermore, there should be a relationship between incremental rateof penetration and the differential pressure existing between the mud columnand the formation. Knowledge of such a relationship would provide a method tomaintain minimum differential pressures and thus improve drilling efficiency, andpredict formation pressures while drilling.