Thomas W. Engler

Methods of Using Logs to Quantify Drillability

Correlations between sonic logs and the formation drillability for different lithology types have been developed from data taken from 10 wells in North America. The gamma ray log was used in conjunction with drilling data to calculate the drillability. The drillability from penetration rate models is back calculated from bit design and reported field wear in conjunction with meter by meter operating parameters, formation type and pore pressure. Then this drillability was correlated with sonic logs for different lithologies as defined by the gamma ray log. The different formation types clearly show different correlations for the normalized correlations between drillability and sonic logs. The non-homogeneous lithologies are also correlated and normalized to rock strength from the sonic logs where the percent formation type mixtures are determined from the gamma ray. Data from multiple wells is presented showing the accuracy of the presented approach where more then 100,000 data points were statistically analyzed and evaluated in the development of the equations presented herein. The drillability from inverted penetration rate models has been verified to give good representation of rock strength based on comparison with triaxial laboratory data and makes the use of this model more versatile. The correlations provide improved estimations of rock strength which can be used in drilling performance simulation and wellbore stability studies.

A New Approach to Gas Material Balance in Tight Gas Reservoirs

Gas material balance in conventional, volumetric reservoirs is described by a linear relationship between pressure/z-factor (p/z) and cumulative production. Unfortunately, tight gas reservoirs do not exhibit this type of behavior, but instead develop a nonlinear trend, which is not amenable to conventional analysis. The nonlinearity is a function of two items: the testing method (time) and the reservoir characteristics. For these type of reservoirs the testing time is insufficient to reach average reservoir pressure and from a practical viewpoint, it is not possible to shutin for extended periods of time. Therefore, a method has been developed to use key intersection points and slopes from a tight gas material balance plot to better understand the reservoir behavior. This work begins by explaining the nature of the nonlinear trend in terms of flow regimes. The primary objective is to improve the estimate of gas-in-place and recovery in a tight gas reservoir. Typically, gas-in-place is underestimated using conventional techniques. It is demonstrated that by using the appropriate slope with the initial pressure an improved (increased) estimate of gas-in-place is achieved. Furthermore, it is possible to distinguish the effect of infill wells and subsequently determine the incremental recovery. Included are field examples from the San Juan Basin and southeast New Mexico, which demonstrate the technique.