An Assessment of the Mechanical Stability of Wells Offshore Nigeria
- J.P. Lowrey (Mobil E and P Services Inc.) | Steiner Ottesen (Mobil R and D Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1995
- Document Type
- Journal Paper
- 34 - 41
- 1995. Society of Petroleum Engineers
- 1.7 Pressure Management, 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.1.6 Hole Openers & Under-reamers, 1.6.1 Drilling Operation Management, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.6.1 Drilling Operation Management, 1.1 Well Planning, 4.1.5 Processing Equipment, 1.6.10 Running and Setting Casing, 5.6.1 Open hole/cased hole log analysis, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11 Drilling Fluids and Materials, 1.6.9 Coring, Fishing, 3 Production and Well Operations, 1.12.6 Drilling Data Management and Standards, 1.6 Drilling Operations, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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In 1991 lost time due to stuck pipe related drilling problems accounted for approximately 18% of total drilling time in Mobil Producing Nigeria Ultd.'s (MPN) offshore operations. The primary cause of stuck pipe was identified as mechanical wellbore instability. This paper presents an assessment of the mechanical stability of MPN's wells offshore Nigeria. The objectives of the study were to: 1) determine the magnitude of the in-situ principal stresses and material properties of the troublesome Intra-Biafra and Qua Iboe shale sequences; 2) quantify the drilling fluid densities required to drill mechanically stable wells through these mechanically weak formations; 3) review and recommend well planning and operational parameters which aid in minimizing wellbore stability-related drilling problems. The wellbore stability assessment was carried out with the aid of a three-dimensional computer simulation model using field derived data from a number of wells in the study area to corroborate the results. The collection and analysis of drilling data (borehole geometry and density logs, pore pressure, extended leak-off tests, local geology and other relevant well records) to determine the magnitude of the in-situ principal stresses, together with uniaxial and triaxial compressive strength measurements on formation cores are discussed. Minimum safe drilling fluid densities to promote wellbore stability as a function of well geometry and depth are presented for the most troublesome formations drilled in the study area. Implementation of the results into well planning and operations reduced wellbore stability related problems and associated trouble time to less than 5% in 1992.
Oil Mining Leases (OML) 67 and 70 lie in the southeastern portion of the Niger river delta, offshore Nigeria (Fig. 1). The leases are operated by MPN, and their joint venture partner is the Nigerian National Petroleum Corporation (NNPC). Numerous hydrocarbon-bearing structures exist within the study area, some of the largest of which are the Edop, Ubit, Oso and the Iyak fields (Fig. 2). Commercial hydrocarbons occur in sandstone reservoirs of the Miocene and Pliocene. A generalized lithostratigraphic column for the southeast Niger delta is shown in Figure 3.
The geology of the area is characterized by massive unconsolidated sands of the Pleistocene Benin Formation at surface, overlying the predominately sandy D-1 member of the Pliocene Agbada formation. Below the D-1 is the Qua Iboe member of the Agbada; predominately a weak shale which is prone to mechanical instability. Some intercalated sands are encountered toward the bottom of the Qua Iboe. The Qua Iboe is sometimes absent near onshore (northern half of OML 67/70), having been eroded. Below the Qua Iboe are found the Rubble Beds, so named due to the fact they are comprised of eroded sediments of the Upper Biafra. The Rubble Beds are also sometimes absent near onshore.
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