Mechanical Earth Model Calibration for Sanding Potential Estimation: A Case Study from the Tombua-Landana Development in Angola's Deepwater Block 14
Authors: Ray Peng , Hilarides William , Wetzel Rodney J. , Sweep Miles Norman 
Published: February 2016

Abstract
Sand control risks, costs, and alternatives are significant factors in the planning phase of a field development. A mechanical earth model (MEM) which includes in-situ stress and rock strength should be constructed and used to evaluate potential for rock failure during production. Fully integrating and calibrating strength, stress, and failure models is crucial to correctly characterize sanding potential. The MEM for the Tombua-Landana Development in deepwater Block 14, offshore Angola will be presented to illustrate the methodology of this approach. In this field case:
In addition to conventional triaxial core tests which provide rock strength in a limited number of samples, core scratch test data was used to establish continuous rock strength over most cored intervals. A log based neural network was used to extend the rock strength information to the non-cored intervals. Comparison between log, scratch test, and triaxial core test strengths will be presented.
Acoustics based stress computations are calibrated to leak-off test and mini-frac stress measurements.
Calibration of failure models to both drilling wellbore stability and completion safe drawdown pressures verifies the validity of the mechanical earth model.
Both openhole and cased and perforated sanding potential were evaluated using the fully integrated earth model. Oriented perforations have the potential to significantly reduce the risk of sanding in completions without gravel packs or screens.

Introduction
The Tombua-Landana field is located in Block 14, offshore Angola with water depth approximately 800 to 1,300 ft. The Tombua and Landana central reservoirs are composed of high quality sands deposited in a deepwater slope valley environment on the northwestern flank of the Congo River Fan. The producing reservoirs are Lower Miocene CN3 in age and form moderately thick successions of sand sequences. The overall Tombua-Landana depositional system comprises a series of large, stacked, offset channels.
Tombua-Landana development is the third major field development in Angola's Block 14, after Kuito and Benguela-Belize-Lobito-Tomboco. It is a major capital project that will have expenditures in the order of billion dollars for all components. Geomechanical modeling played an important role during the early phase of field development planning. Completion decisions are aided by an accurate assessment of sanding characteristics of various well designs.
A MEM was constructed to estimate sanding potential for the Tombua-Landana field. This MEM was then calibrated to field data. An overview of the building of the Tombua-Landana MEM is presented in this paper.

Building a Mechanical Earth Model (MEM)
A MEM consists of in-situ stresses and rock strength linked with a failure model. These are all calibrated to wellbore observations of rock failure behavior. Sand production results from rock failure caused by the imbalance between the local stress state near the wellbore and rock strength.
In this section, the process and method of determining in-situ stresses and rock strengths will be shown. It will also be shown that the sand production predicted using the in-situ stresses and rock strengths matches the core tests, well test results, and drilling performance. Therefore, the in-situ stresses, rock strengths, and failure model are most likely valid and can be used for the field sanding potential analysis.

Earth In-Situ Stresses
The in-situ stress magnitudes and orientations can affect the sanding potential of wells. In most cases, principal in-situ stresses can be expressed as:Overburden stressMaximum horizontal stress (SHmax)Minimum horizontal stress (Shmin)
The overburden stress is in the vertical direction and is generally, as in this case, calculated by vertically integrating the density log.
Two horizontal stresses are the two principle stresses perpendicular to each other on a horizontal plane. The larger horizontal stress is called maximum horizontal stress and the lesser one is called minimum horizontal stress.