Porosity Calculation of Horizontal Wells when Acoustic Slowness Is Abnormal - A Case Study in Northern Ordos Basin, China

Owing to the complicated borehole environment and detection characteristics of acoustic log, acoustic slowness is always partial distortion in the horizontal wells. Besides, the logging data is sometimes insufficient due to the expensive cost and imperfect technology. Therefore the porosity calculation with insufficient and partial wrong horizontal logging data becomes one of the big challenges. Such as in the Shiguhao Area which is located in northern Ordos basin, only several logs have been measured. Through the horizontal well log interpretation and formation evaluation work, we found that the acoustic slowness was abnormal in some formations by the stratigraphic correlation. Then, several factors which lead to the high acoustic slowness are summarized in this paper. For the porosity calculation in the horizontal wells, the abnormal acoustic slowness is not recommended to be used because it can be so hardly corrected that may reduce the accuracy of the calculation since others always get the defective result by acoustic correction. Meanwhile, due to the lack of density log, we propose the use of compensated neutron log instead of abnormal acoustic log to calculate porosity. Several case studies are in great consistent with oil test results which confirm the feasibility of our method.