Analyses of Sonic Data in an Indonesian Well for Formation Damage, Stresses, and Bedding

摘要

To help design the completion of an Indonesian developmentrnwell, we have carried out cross-dipole dispersion analysesrnover a depth interval of approximately 500 ft. Dispersionrnanalyses provide estimates of radial extent of formationrndamage and indicators of stress- and bedding-inducedrnanisotropies. Most of these sections of this vertical wellrnexhibit sand and shale lithology with shear slownessrnanisotropy ranging from 5 to 10%. Above the angularrnunconformity in the deeper section of the well, a clean sandrninterval (Sand A) shows evidence of cross-dipole dispersionrncrossovers. Crossing dipole dispersions are indicators ofrnstress-induced anisotropy dominating the sonic data. We haverndeveloped a new technique for estimating the maximumrnhorizontal S_H,and minimum horizontal S_h stress magnitudesrnusing multi-frequency inversion of wideband cross-dipolerndispersions. At the mid-point of sand A, we estimate thernoverburden stress S_V= -2278 psi; the maximum horizontalrnstress S_H= -1843; and minimum horizontal stress S_h= -1698.rnThe fast shear direction is NW5 in this depth interval. Radialrnprofiling of formation shear velocity indicates varying degreesrnof mechanical alteration extending from one to two boreholerndiameters in the entire depth.rnA second clean sand interval B, 300 ft above sand A, showsrndipole shear anisotropy on the order of 10%. However, crossdipolerndispersions appear to merge together at highrnfrequencies instead of showing a cross-over. Radial profilingrnof shear velocity in the two orthogonal directions confirmsrnmechanical damage extending to about 2x the boreholerndiameter. The near-wellbore region in this interval appears tornhave deformed (material creep) in an attempt to reduce shearrnstresses and attain hydrostatic equilibrium. Using the samernstress sensitivity coefficients as estimated in the lower sand A,rnthe differential stress (S_H - S_h) is estimated to be about 20%rnlarger in the upper interval, than the corresponding value inrnthe lower interval.rnThe fast-shear direction varies abruptly across the angularrnunconformity, changing from NW5 to NW75. Below thernunconformity, this section exhibits beds with dips rangingrnfrom 10° to 30°. Cross-dipole dispersions show significantrnanisotropy and are non-intersecting at higher frequencies.rnNon-intersecting dispersions indicate bedding-inducedrnanisotropy dominating the cross-dipole data. We haverninverted borehole sonic velocities for four combinations of thernTI-shale anisotropy, which can be combined with walk awayrnVSPs to obtain all the shale anisotropy constants. Thesernconstants are needed in generating synthetic AVO gathers inrnanisotropic shale formations. Quantitative estimates of thernradial extent of near-wellbore damage in this well suggest thatrnperforations should penetrate deeper than twice the boreholerndiameter to avoid potential permeability impairment caused byrnnear-wellbore mechanical damage.