Estimating Formation Tops While Drilling Using Rate of Penetration (ROP) and Mechanical Specific Energy (MSE)

Abstract

Abstract Accurate determination of formation tops while drilling is a critical part of exploration geology workflow. Operational decisions on coring, wireline logging, casing and final well depth largely depend on it. One of the commonly used methods for picking formation tops while drilling is to correlate rate of penetration (ROP) of the new well to wireline logs from offset wells where there is no logging while drilling (LWD) data. Placing formation tops based on ROP only can result in picking the wrong formation tops. To improve the workflow and outcome, this paper proposes the combination of ROP and Mechanical Specific Energy (MSE) for estimating formation tops while drilling. MSE is a measure of energy required to crush or drill through a unit volume of rock. Because MSE is related to rock strength, it can be correlated to changes in lithofacies and formation tops. Three key steps necessary for utilizing mechanical specific energy to estimate formation tops. The first step involves selection of input drilling data relevant to the applicable MSE equation. There are several empirical equations in the literature which can be used for estimating MSE. Input data are ROP, Weight on Bit (WOB), Bit Size (BS), Rotation Per Minute (RPM), and Torque (TORQ) from both the offset wells and the new well. Second, it is necessary to utilize predetermined empirical equation to estimate MSE. Third, correlate MSE and ROP from the new well to MSE, ROP and wireline logs from offset wells (where available) in order to determine formation tops in the new well. Application of the proposed workflow to two wells shows 1) distinct bed boundaries, which agree with formation tops picked using wireline logs; (2) that including MSE increases confidence and reliability of the data and makes it easy to identify the different formation boundaries based on the observed features of both MSE and ROP in the new well; and (3) that MSE variations are sensitive to formation strength, which may indicate rock mechanical changes and formation heterogeneity. This paper presents an alternative method of picking formation tops using MSE and ROP while drilling. The preliminary results based on the two test wells showed over 95% match with those picked using wireline logs of the same new well. As a result, this workflow enhances the ability of geoscientists to correlate subsurface geological features, reduces the uncertainty associated with picking formation tops, casing and coring depths. Furthermore, it improves the confidence in the result of picking formation tops, enhances quality of operational decisions and reduces non-productive time (NPT) and well-cost.