As a borehole is drilled, hydraulic pressure of the drilling mud must replace the support lost by removal of the original column of rock. But mud pressure being uniform in all directions cannot exactly balance the earth stress. Consequently, rock surrounding the borehole is distorted or strained, and may fail if the redistributed stresses exceed rock strength. The bottom line for the drilling engineer striving to maintain stable hole is choosing the right mud weight.
The aim of this study is to build a Mechanical Earth Model using available data for one of the Iranian oil field to evaluate wellbore stability and to choose optimum mud weight.
For making MEM different sources of data including, drilling data, logging data, DST, MDT, LOT, FIT, etc were used. After making MEM with available data, suggestion for selection the right mud weight for each interval to prevent the common instability problems (lost circulation, borehole breakout) was made.
MEM for this field can now be used to predict not only the safe mud weight window and possible drilling hazards, but can also be used for studies like sand production and perforation stability and so on.
Numerical modeling has been carried out using MEM data for comparing analytical and numerical wellbore stability analysis to select optimum mud weight, to investigate the feasibility of managed pressure drilling for problematic formations, and to determine the most effective azimuth for drilling horizontal wells into the reservoir section.
Mechanical Earth Model (MEM) is a numerical representation of the state of stress and rock mechanical properties for a specific stratigraphic section in a field or basin.1 Many oilfield projects are challenging because of geomechanical problems arising from overpressure, wellbore instabilities, reservoir compaction, casing failure, sanding, surface subsidence, fault reactivation etc.2 Minimizing the risk of problems related to geomechanical properties requires understanding the geomechanics of well construction and field. Building MEM can drastically reduce time and cost of field development.
Some applications of MEM which have been implemented in oil industry are as follows:
Prediction of pore pressure and fracture gradient to design appropriate casing program which reduce significantly the cost of materials and rig time.1
Risk reduction of stuck pipe incidents due to wellbore instability that may cause lost BHAs and subsequently increasing NPT for freeing pipe, performing additional wiper trips and hole cleaning and side tracking.1
Selection of proper completion, maintenance of perforation stability and safe drawdown pressure to avoid sanding in exploration and development wells.3, 4
Investigation of pore pressure drop on reservoir compaction due to depletion that causes an increase in the effective stress acting on the reservoir. The change in stress may lead to problems such as: reservoir compaction and plastic deformation of rocks, surface subsidence, reduction in porosity and permeability, casing deformation, reactivation of faults or bedding-parallel slip.2
Making real-time decision while drilling with 3D MEM analysis to reduce drilling risks. In this real-time process the MEM is updated by downhole measurements, LWD and
MWD data and the drilling plan is made based on revised MEM.5, 6
Here the methodology of building a MEM is presented. Generally geomechanical model relates dynamic elastic properties to static equivalents. These elastic static properties are then used to characterize formation strength and in-situ stress.3 The MEM consists of depth profiles of elastic or elasto-plastic parameters, rock strength, earth stresses, pore pressure and stress direction. The sources of information which has been used to build an MEM are shown in Table 1.