Borehole inclination

In preparation for a field wide quick look correlation, all well logs need to be corrected for borehole inclination. This is done routinely with software which uses the measured depth below the derrick floor ( alonghole depth below derrick floor AHBDFor measured depth , MD) and the acquired directional surveys to calculate the true vertical depth subsea (TVSS). This is the vertical distance of a point below a common reference level, for instance chart datum (CD) or mean sea level (MSL). Figure 5.41 shows the relationship between the different depth measurements. 

Every accelerometer has a response curve of the type shown schematically in Figure 4-222. Instead of having an ideal linear response, a nonlinear response is generally obtained with a skewed acceleration for zero current, a scale factor error and a nonlinearity error. In addition, the skew and the errors vary with temperature. If the skew and all the errors are small or compensated in the accelerometer s electronic circuits, the signal read is an ideal response and can be used directly to calculate the borehole inclination. If not, modeling must be resorted to, i.e., making a correction with a computer, generally placed at the surface, to find the ideal response. This correction takes account of the skew,... 

Only surface WOB and torque are available. Borehole inclination is 40 . 

Diameters of the holes vary from 5—25.4 cm. Drilling perpendicularly to the deposit is preferable but ia folding or tilted beds inclined drilling is often practiced. Spaciag of the holes and borehole diameters depend on the hardness and fracturiag characteristics of the stone, and desired top size for the primary cmsher. 

As drilling technology moved the pursuit of hydrocarbon resources into higher-cost offshore and hostile environments, intentionally deviated boreholes required information such as azimuth and inclination that could not be derived by surface instruments. Survey instruments, either lowered on a sand line or dropped into the drill pipe for later retrieval, to some degree satisfied the requirements but consumed expensive rig time and sometimes produced questionable results. 

Compute the inclination of the borehole. Are the accelerometers working properly Why ... 

In classical geosteering the sensors for inclination, azimuth, drilling parameters, and logging are located above the mud motor and the distances may be in the order of those shown in Figure 4-296 that is 30 ft or more above the drill bit. Although radial measurements can be performed to verify that the borehole is being drilled in the pay zone, it is often too late to make a correction and the borehole leaves the pay zone. 

Borehole directional survey Refers to the measurements of the inclinations, azimuths and specified depths of the stations through a section of borehole. 

Corrective jetting runs Action taken with a directional jet bit to change the direction or inclination of the borehole. 

Dogleg severity A measure of the amount of change in the inclination and/ or direction of a borehole usually expressed in degrees per 100 ft of course length. 

Hole curvature Refers to changes in inclination and direction of the borehole. 

Original hole direction and inclination is N20°E and 10°, respectively. It is desired to deviate the borehole so that the new hole inclination is 13° and direction N30°E. 

INCORPORATING CHEMICAL EFFECTS IN A POROTHERMOELASTIC FORMULATION AND APPLICATION TO INCLINED BOREHOLES... 

Abstract A poromechanics formulation for transversely isotropic chemically active poroelastic media under non-isothermal conditions is presented. The formation pore fluid is modeled as a two-species constituent comprising of the solute and the solvent. The model is applied to study the thermo-chemical effects on the stress and pore pressure distributions in the vicinity of an inclined borehole drilled in a chemically active transversely isotropic formation under non-isothermal conditions. 

The coupling effects of various poromechanical processes on the response of a porous medium have been successfully addressed by Biot s theory of poroelasticity and its extensions [3,4,5,8,2], The chemical effects have also been addressed by considering interaction between the porous matrix and a pore fluid comprising of a solute and solvent [10, 7, 6], Comprehensive anisotropic poromechanics formulations and corresponding solutions for the inclined borehole problem have been presented [4—2], However, the coupled chemo-thermo-hydro-mechanical response of an anisotropic porous medium has not been addressed to date. 

The borehole is assumed to be infinitely long and inclined with respect to the in-situ three-dimensional state of stress. The axis of the borehole is assumed to be perpendicular to the plane of isotropy of the transversely isotropic formation. Details of the problem geometry, boundary conditions and solutions for the stresses, pore pressure and temperature are available in [7], The solution is applied to assess the thermo-chemical effects on stresses and pore pressures. Both the formation pore fluid and the wellbore fluid are assumed to comprise of two chemical species, i.e., a solute fraction and solvent fraction. The formation material properties are those of a Gulf of Mexico shale [7] given as E = 1853.0 MPa u = 0.22 B = 0.92 k = 10-4 md /r = 10-9 MPa.s Ch = 8.64 x 10-5 m2/day % = 0.9 = 0.14 cn = 0.13824 m2/day asm = 6.0 x 10-6 1°C otsf = 3.0 x 10-4 /°C. A simplified example is considered wherein the in-situ stress gradients are assumed to be trivial and pore pressure gradients of the formation fluid and wellbore fluid are assumed to be = 9.8 kPa/m. The difference between the formation temperature and the wellbore fluid temperature is assumed to be 50°C. The solute concentration in the pore fluid is assumed to be more than that in the wellbore fluid such that mw — mf> = —1-8 x 10-2. 

A transversely isotrpoic porochemothermoelastic model has been presented in this paper. The model has been applied to the problem of an inclined borehole and the thermo-chemical effects on stress and pore pressure distributions have been isolated in a simplified example. Results from the simplified example show that a higher temperature along with a lower solute concentration of the wellbore fluid results in increased magnitudes of the pore pressure near... 

Abousleiman, Y. and Ekbote, S. (2005) Porothermoelastic solution for an inclined borehole in transversely isotropic media. J. Appl. Mech. 72, 102-114... 

Ekbote, S. and Abousleiman, Y. (2005) Porochemoelasticity Application to the inclined borehole. J. Engrg. Mech., ASCE, in press... 

The comparison is plotted in Figure 4 for 60° inclined boreholes (BH 147, 148, 154, 155, 178 and 179). The noticeable spread of the results may be attributed to the local rock mass heterogeneities, such as fractures. With the exception of the very early time, simulated displacements are within the range of the measured ones. The best agreement is obtained for Anchors 2 and 3, whereas the... 

Figure 4. Measured and LBNL s simulated displacements for 60 ° inclined boreholes.

Figure 7 shows that LBNL s elastic and CEA s elasto-brittle models are within the range of maximum and minimum measured values, except in areas close to the drift wall. Near the drift wall, there appears to be an additional shift in the displacement field, possibly caused by inelastic behaviour near the open wall surface. The CEA s ubiquitous joint model overpredicts the displacement in the 60° inclined boreholes (Figure 7a) possibly by an overprediction of near-wall inelastic deformations. 

The borehole is assumed to have a long straight section, which can be vertical or inclined, and the borehole axis may not align with any of the principal in-situ stresses or with the direction normal to the bedding planes. Under some conditions, cross-sectional planes which are initially perpendicular to the borehole axis before drilling may no longer be planar after creation of the borehole and a 3-dimensional model is required. 

Depth The vertical depth below the surface in the case of incline shafts and boreholes it may mean the distance reached from the beginning of the shaft or hole, the borehole depth, or the inclined depth. 

A very distinct cluster of approximately 800 events from the boundary of rock salt and anhydrite was observed within a time period of one day. These events form a planar ring structure with a diameter of approximately 10 m. Fig. 11.15 (a) displays a plan view onto the plane of the cluster in a rotated coordinate system. A second cluster but with temporally dispersed activity occurred 20 m above the previous cluster (Fig. 11.15 b). Both clusters are planar and show approximately the same orientation - which is in agreement with the general inclination and strike of the geological layers, see Fig. 11.14. By drilling of two boreholes into the center and the pe-... 

The chemical analyses of the drilled cores can be stored section by section, with associated data relating to the co-ordinates of the borehole, the depth and the thickness of the deposit. By making use of appropriate programs it is moreover possible to store the results obtained from inclined boreholes and from trial pits and, with due regard to the dip of the strata, to obtain a strata-related representation of the geochemical conditions. 

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