Improved oil recovery from chalk reservoirs by water‐flooding may cause mechanical weakening and change in elasticity. Confined compressive strength testing of chalk from a North Sea reservoir was done in water‐saturated and oil‐saturated conditions. During testing, elastic wave velocities were sampled by ultrasonic transducers, so that subsequently Biot's coefficient could be modelled. The porosity declined via an ‘elastic phase’, a ‘transitional phase’, an ‘elastoplastic phase’ and a ‘strain hardening phase’, but Biot's coefficient indicates that these terms may be partly misleading. In the ‘elastic phase’, porosity and Biot's coefficient decrease, indicating elastoplastic deformation. In the ‘transitional phase’, Biot's coefficient increases as a reflection of breaking contact cement (pore collapse), whereas Biot's coefficient remains stable in the ‘elastoplastic phase’, indicating elastic deformation on the virgin curve. Plastic deformation takes place during phases of creep, where both porosity and Biot's coefficient decrease. Similarly, in the ‘strain hardening phase’, both porosity and Biot's coefficient decrease as a reflection of elastoplastic deformation. For chalk with 45%–47% porosity, the ‘transitional phase’ begins at 8 MPa axial stress when water‐saturated and at 12 MPa when oil‐saturated. For chalk with 41%–43% porosity, the corresponding stresses are 16 and 20 MPa. For chalk with 32%–36% porosity, the corresponding stresses are 23 and 31 MPa. Chalk samples with irreducible water saturation and movable oil were water‐flooded. They yield at stresses close to corresponding oil‐saturated samples, but after flooding show compaction trends not significantly different from the water‐saturated samples. Water‐flooding promotes pore collapse as reflected in an increasing Biot's coefficient. The consequent softening effect on acoustic impedance is small as compared with the effect of increasing fluid density. With respect to 4D seismic, water‐flooding causes distinctly higher acoustic impedance and Poisson's ratio irrespective of compaction.

中文翻译：

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储层粉笔的注水和固结-对孔隙度和比奥系数的影响

通过注水改善白垩储层的采油率，可能会导致机械强度减弱和弹性变化。在水饱和和油饱和的条件下，对北海储层的白垩粉进行了密闭抗压强度测试。在测试过程中，通过超声波换能器对弹性波速度进行采样，从而可以对比奥系数进行建模。孔隙率通过“弹性相”，“过渡相”，“弹塑性相”和“应变硬化相”降低，但比奥特系数表明这些术语可能会造成部分误导。在“弹性相”中，孔隙率和比奥系数降低，表明弹塑性变形。在“过渡相”中，毕奥特系数随着接触水泥的破坏（孔隙塌陷）而增加，而毕奥特系数则增加。s系数在“弹塑性阶段”保持稳定，表明原始曲线上发生了弹性变形。塑性变形发生在蠕变阶段，孔隙率和比奥特系数均降低。同样，在“应变硬化阶段”，孔隙率和比奥特系数均降低，反映了弹塑性变形。对于孔隙率为45％至47％的白垩，“过渡相”在水饱和时从8 MPa轴向应力开始，在油饱和时在12 MPa时开始。对于孔隙率为41％–43％的白垩，相应的应力为16和20 MPa。对于含32％的粉笔 孔隙率和比奥特系数均降低，反映了弹塑性变形。对于孔隙率为45％至47％的白垩，“过渡相”在水饱和时从8 MPa轴向应力开始，在油饱和时在12 MPa时开始。对于孔隙率为41％–43％的白垩，相应的应力为16和20 MPa。对于含32％的粉笔 孔隙率和比奥特系数均降低，反映了弹塑性变形。对于孔隙率为45％至47％的白垩，“过渡相”在水饱和时从8 MPa轴向应力开始，在油饱和时在12 MPa时开始。对于孔隙率为41％–43％的白垩，相应的应力为16和20 MPa。对于含32％的粉笔–孔隙率为36％，相应的应力为23和31 MPa。将水饱和度和还原油含量降低至不可减少的粉笔样品进行水驱。它们在接近相应的油饱和样品的应力下屈服，但注水后的压实趋势与水饱和样品没有显着差异。注水增加了比奥系数，这反映了孔隙的塌陷。与增加流体密度的效果相比，对声阻抗的相应软化效果很小。对于4D地震，无论是否压实，注水都会导致明显更高的声阻抗和泊松比。