Method Of Estimating In-situ Stresses

The distribution of stresses controls the formation, migration and accumulation ofoil and gas, it is significant for the exploration and development. The mechanicscharacteristics of coal, low Young’s modulus and high Poisson’s ratio, make thenormal stress prediction method cannot be applied directly. Aiming at this problem,this paper based on triaxial compression and Kaiser experiment, proposed a new wayto predict coal seam stresses using hydraulic fracturing and logging data.Zhengzhuang coalbed methane field was taken as an example to verify this method.By analyzing array acoustic logging and conventional logging data, the models ofsand and coal seam shear wave time were established adopting longitudinal wave,gamma, deep resistivity, density and neutron logging. Carried on the calculationresults fitting to the experimental findings, the linearity is up to0.816and0.723. Thenew models took the effect of mineral content, hydrogen index and compaction intoaccount, provided the underlying data for prediction of in-situ stresses.Through the experiment, this paper introduced the deformation parameter, on theassumption that stress distribution has line relationship with the strains on the coal,the stresses prediction model is established. To calculate, logging and fracturing datais necessary, and in the new model, redistribution of vertical stress is revised, theeffect of pore pressure and the stress of structure was considered as well.The deformation can explain the phenomenon that in the same geologicalbackground, different lithologies exhibit different stress, considering the stressconsumption due to deformation. The model can be used for coal seam successfully.With hydraulic fracturing data, it can give a good explanation for the distribution ofstresses of complex tectonic belt and different rocks.By using the stresses method, the stresses of Zhengzhuang coalbed methane fieldwas predicted. Comparing with the roof and floor, coal seam has the low elasticmodulus, higher Poisson’s ratio and low stress value; the vertical stress distributesbetween10.61~22.38MPa, minimum horizontal stress distributes between7.27~23.3MPa, maximum horizontal stress distributes in9.08~29.12MPa.The calculation shows that the area can be divided into three tectonic units:①theweaker away from the Sitou fault belt, residual tectonic stress is3.62MPa, state ofstress is σv> σH> σhand the critical depth is about650m;②the stronger near theSitou fault belt, residual tectonic stress is7.13MPa, state of stress is σH> σV≥σhandthe critical depth is about800m;③the strongest, residual tectonic stress is10.35MPa,state of stress is σH> σh≥σVand critical depth between1150~1400m.Based on the results of calculation, geological and engineering factors, the strongeris considered as the best area for exploition; the weaker is worse; and the strongest isthe worst.