Drilling Time Reduction Through an Integrated Rock Mechanics Analysis

Abstract

Abstract A working methodology to minimize wellbore stability problems has beenestablished through the use of unique laboratory tests, an experimentaldatabase for fluid-rock interaction and physical properties of shales, and anintegrated modeling approach utilizing different types of experimental andfield data. The model simulations provide output accounting for a wide range ofinput parameters such as well inclination, mud chemistry, rock mechanicalproperties, field stresses and pressures, formation anisotropy and shalemineralogy. The model output can subsequently be used to diagnose fielddrilling problems or to design drilling operations. As an example, data from anHP/HT field offshore Mid-Norway as well as a field in the overpressured shalesin the southern part of the Norwegian North Sea have been analyzed andcompared. Introduction Wellbore stability problems while drilling through shales are widely knownas a primary factor increasing drilling time and costs. In order to avoid suchproblems, the drilling process is designed to minimize the risk of wellboreinstabilities like tight hole/stuck pipe, kicks and mud losses. Such a design includes balancing the mud weight between the collapse andfracture pressure, optimizing the well azimuth and inclination with respect tostresses and mechanical properties, and improving drilling procedures toachieve good hole cleaning and to minimize variations in the mud pressure fromequivalent circulating density (ECD) and surge/swab effects. Moreover, the mud composition can also be used actively to enhance wellborestability. Often, an oil based mud (OBM) is preferred since it may seal moreeffectively towards the pore fluid, supporting the low-permeable borehole wallbetter. However, due to for instance environmental constraints or due tosagging problems in high pressure/high temperature (HPHT) wells, water basedmuds (WBM) sometimes have to be applied. In this case, communication betweenmud and pore fluid may reduce the effective support from the mud, leading tostability problems. However, adding salts like KCl or CaCl2 to thewater phase may improve the wellbore stability through chemicaleffects1–3. Such effects are effectively governed by diffusiveprocesses. This results in time-dependent wellbore stability in the sense thatan initially stable borehole may turn unstable after a certain period of time, depending upon formation and fluid characteristics. Thus, it may be of utmostimportance to optimize the mud composition for a given well. In order to achieve a good drilling practice, well design and mud design, vital data to evaluate the risk of hole instabilities need to be acquired. Thismay be achieved through extensive field data acquisition and/or use ofexperience data and data-bases in combination with well stability and wellborecleaning analysis. Such efforts may greatly reduce drilling problems and savecosts4. It is the aim of this work to demonstrate the value of different tools andtechniques to provide data needed and perform adequate data integration andwellbore stability analysis when drilling through shales. Our methodology isbased on more than 15 years of research and development within the field. Aspart of this, specialized laboratory tests and computation models have beendeveloped. Finally, in order to provide realistic examples of use of themethodology, data from a HP/HT field offshore Mid-Norway as well as a field inthe overpressured shales of the southern part of the Norwegian North Sea havebeen applied to the methodology. Methodology for Wellbore Stability Analysis In order to perform an integrated rock mechanics analysis of the wellborestability in terms of estimating the stable mud weight window and its timedependence, a number of effects and a corresponding number of parameters haveto be accounted for as already discussed. These parameters are related to theformation properties, the formation conditions, and the wellbore data. We have developed a numerical model that accounts for a variety of suchparameters, including chemical effects of both osmotic and ionic exchangenature, as well as mechanical plasticity and anisotropy. This model haspreviously been described in Fjær et al1.