| Chad Oilfield is an important reserve increase area in Africa.Practical drilling analysis shows that downhole accidents such as well leakage and collapse often occur during shallow drilling in Chad Oilfield.It is urgent to use multidisciplinary method to study the formation three pressures(pore pressure,collapse pressure and fracture pressure)in order to provide scientific basis for optimum drilling design,Mainly carried out are:?Correction of shallow acoustic time difference and density logging curve;?Calculation of shallow petrophysical parameters and establishment of stratigraphic lithologic profile and identification of fluid properties;?Logging calculation of rock mechanics parameters,in-situ stress and formation three pressures:?Determination of density window of safe drilling mud and evaluation of borehole wall stability.The following achievements and understandings have been achieved:(1)In view of the response characteristics of shallow well logging curves which are on the high side or on the low side,undercompaction of sonic time difference curves,correction of borehole environmental impact and correction of borehole environmental impact of density logging curves are carried out.The corrected curves reasonably reflect the actual situation of shallow formation.(2)Based on the "four properties" research,the calculation model of shallow petrophysical parameters is established through logging data and core experiment analysis data.Comparing different calculation methods of shale content,porosity and permeability,it is found that calculating shale content based on natural gamma logging curve has high accuracy;The porosity model based on regression fitting of acoustic time difference and density logging curve has high correlation;The permeability calculation model based on porosity fitting is reasonable and feasible;The calculation model of oil saturation based on non-electric gamma-density logging curve has good effect;The parameter model is programmed into CRA program to identify formation lithology,and the volume analysis section of formation rock is established to provide lithological basis for calculating rock mechanics parameters,and the fluid properties of formation are identified by logging response characteristics,which provides a lithophysical basis for calculating rock mechanics parameters.(3)The normal compaction trend line of Chad oilfield shallow layer is constructed by sonic time difference curve,and it is found that shallow layer is in normal or undercompacted state.The shear-wave time-difference curve is constructed by using the P-wave and S-wave time-difference relationship of key wells in the work area.Through the conversion relationship between experimental data and logging data,the conversion model of dynamic and static rock mechanics parameters is obtained,and the rock mechanics parameters profile of the work area is established.(4)Formation pore pressure is calculated by AC-DEN-Rt method and improved Eaton method respectively.Comparisons show that the improved Eaton method has higher accuracy and can be used as a pore pressure calculation model in this area.By comparing the in-situ stress calculated by Anderson model and depth fitting model,it is found that the in?situ stress calculated by Anderson model is more accurate.(5)Based on Mohr-Coulomb criterion and rock fragmentation index,a new model for calculating shallow collapse pressure is established;a new model for calculating fracture pressure is established based on SHEAR-TENSILE failure criterion and maximum tensile stress theory,and a shallow safe drilling mud density window is determined by using the established three-pressure profile.(6)Based on Forward platform and using Fortran language to program,a window program for calculating shallow three pressures and safe drilling mud density is compiled,and logging data of several wells are processed.The program runs steadily and the results are reasonable and reliable,which provides a rock mechanics basis for shallow safe and efficient drilling in Chad. |
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