Porosity Stress Sensitivity Of Tight Sandstone

In the process of well-drilling and exploration,abnomally high pore pressure may result in huge risks,therefore it is of great significance to precisely predict pore pressure in target formation.Recently there exist a popular method to get pore pressure through effective stress,while most of its applications assume porosity effective coffecient to be unity.Wether this simplification is accurate enough is a important part of this paper.In this work,rock samples from blocks of Donghai B tight sandstone gas-field are tested in order to study porosity stress sensitivity as well as porosity effective stress coefficient.In the experiment,effect of stress creep,loading rate and loading/unloading are investigated.Stess-dependent porosity of these rock cores are measured,and a popular porosity-effective stress model are verified.Classification of rock samples by curve fitting of the model associates with that by clay fraction,which means that stress-sensitivity of porosity has someting to do with clay fraction in formation.According to the clay content of the core,the core is divided into three categories,the same core should be in accordance with the effective fitting force model are close to the results,the same core fitting to the same curve,I class,II class,III class core clay content were 2.3-6.9%,7.2-10.9%,14.2-18.0%,the effective stress coefficient respectively.0.815,1.145,2.037..Four clay fraction-effective stress coefficient models are proposed according to clay-rock distribution,namely particle in-particle out model,shell in-particle out model,paricle in-shell out model,shell in-shell out model.According to the model proposed in this paper,when the porosity is located in the interval [0,0.2],clay content is located in the interval [0,0.2],when the clay content is 0,the effective stress coefficient is constant to 0.61.When the porosity is constant,the effective stress coefficient decreases with the increase of porosity.When the porosity is constant,the effective stress coefficient increases with the increase of porosity.The effective stress coefficient is [0.61,2.65].All these models are verified by a case study.The study has found that assuming effective stress coefficient to be unity results in considerable error of the pore pressure when pore pressure are figured out with effective stress,especially in formation with high clay fraction.Accurate effective stress coefficient is proposed to be adopted to get accurate result of pore pressure prediction.