Uncertainty Evaluation of Safe Mud Weight Window Utilizing the Reliability Assessment Method

Abstract

Due to the uncertainty of formation properties and improper wellbore stability analysis methods, the input parameters are often uncertain and the required mud weight to prevent wellbore collapse is too large, which might cause an incorrect result. However, the uncertainty evaluation of input parameters and their influence on safe mud weight window (SMWW) is seldom investigated. Therefore, the present paper aims to propose an uncertain evaluation method to evaluate the uncertainty of SMWW. The reliability assessment theory was introduced, and the uncertain SMWW model was proposed by involving the tolerable breakout, the Mogi-Coulomb (MG-C) criterion and the reliability assessment theory. The influence of uncertain parameters on wellbore collapse, wellbore fracture and SMWW were systematically simulated and investigated by utilizing Monte Carlo simulation. Finally, the field observation of well SC-101X was reported and discussed. The results indicated that the MG-C criterion and tolerable breakout is recommended for wellbore stability analysis. The higher the coefficient of variance is, the higher the level of uncertainty is, the larger the impact on SMWW will be, and the higher the risk of well kick, wellbore collapse and fracture will be. The uncertainty of basic parameters has a very significant impact on SMWW, and it cannot be ignored. For well SC-101X, the SMWW predicted by analytical solution is 0.9921–1.6020 g/cm3, compared to the SMWW estimated by the reliability assessment method, the reliability assessment method tends to give a very narrow SMWW of 1.0756–1.0935 g/cm3 and its probability is only 80%, and the field observation for well kick and wellbore fracture verified the analysis results. For narrow SMWW formation drilling, some kinds of advanced technology, such as the underbalanced drilling (UBD), managed pressure drilling (MPD), micro-flow drilling (MFD) and wider the SMWW, can be utilized to maintain drilling safety.

Study on Method of Determining the Safe Operation Window of Drilling Fluid Density with Credibility in Deep Igneous Rock Strata

“…Credibility. Due to the complexity of the underground geological environment, the incompleteness of data, and the subjective experience of technicians, the calculated drilling geological characteristic parameters have different degrees of uncertainty, and the real values are distributed in a certain range [22,25]. Aiming at the complex geological environment of deep igneous strata with frequent development of intrusions, faults, and fractures, this paper introduces the formation three-pressure prediction method with credibility, which changes the traditional single-value pressure curve into the distribution interval with probability information, thus improving the understanding of the downhole geological environment.…”

Analytical Model of Wellbore Stability Analysis of Inclined Well Based on the Advantageous Synergy among the Five Strength Criteria

“…(3) Calculate the ratio (n i /n j ) of any given two numbers of rock samples that obey different criteria to obtain the relative importance (a ij ) of any given two strength criteria i and j, where the relative importance (a ij ) means the relative importance of strength criterion i to j; its value is determined by the expert according to the ratio (n i /n j ) and the ratio-scaled table (as shown in Table 1). In general, a ij = 1/a ji , a ii = 1, and a ij > 0 (4) Establish the discriminant matrix Λ between different strength criteria using Equation ( 25); then, calculate the eigenvector of the discriminant matrix Λ using Equation (26).…”

“…When a borehole is drilled and formed, the original rock is removed from the formation, so its support effect is replaced by the drilling mud, which will cause stress redistribution near the borehole, and wellbore instability may occur when stress exceeds rock strength. As shown in Figure 1, the mechanical failure of wellbore can be classified as (1) wellbore collapse caused by compressive failure and (2) wellbore fracture caused by tensile failure [25,26]. Too low mud weight (or drilling fluid density) may cause wellbore breakout and even wellbore collapse, while too high mud weight may cause wellbore fracture and even lost circulation.…”

“…Thus, the WSA is vital for designing a proper mud weight to maintain wellbore stability. To accurately predict the collapse and fracture pressures, various analytical and numerical models have been proposed and employed for WSA [8,15,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38], and most of them are still elastic methods. The most common elastic analysis process of wellbore stability includes two major steps [8]: (1) determine the wellbore stress components using analytical solutions of elastic stress distribution and (2) estimate the wellbore failure by selecting a proper strength criterion.…”

Development of a Plugin Based Simulator for Wellbore Stability Analysis and Mud Weight Window Optimization

“…Even though the analytical solution is still relevant for wellbore projects, the conditions required for the solution to be directly applicable (vertical well, no form of coupling, use of MC or the Mogi-Coulomb criterion, linear elastic conditions) may be too restrictive. Recent works tested different methodologies, including the use of Transversely Isotropic constitutive equations to create wellbore models [2] or the addition of uncertainties related to the data available for the analysis, either by considering the spatial variability of the material properties [57,58], employing Reliability Assessment Methods [59] or Quantitative Risk Analysis [6].…”