Deriving Effective Permeability to Improve Productivity Prediction Based on Mobility from Pressure Measurement While Drilling and Relative Permeability Test
Authors: Shi Xinlei , Cui Yunjiang , Xu Sainan , Xu Wankun , Niu Shenyuan , Han Zhilei 
Published: February 2024

Prediction of a well's hydrocarbon productivity depends heavily on a good estimate of permeability. Conventionally, the absolute permeability is estimated from well logs by establishing a porosity-to-permeability (poroperm) relationship using core measurements. However, when there's a lack of representative core samples, this method can lead to inaccurate permeability estimations. More importantly, the effective permeability can reflect the production capacity of the reservoir more than the absolute permeability calculated from the poroperm relationship. In this paper, we propose a method to deriving effective permeability based on mobility from pressure measurement while drilling(PWD). We apply this method to the analysis of an offshore siliciclastic oilfield, where the predicted production by using effective permeability is highly consistent with the actual hydrocarbon production.

In the proposed method, we leverage the rich PWD in our study field. It is found that under the condition of water-based mud drilling, the product of mobility from pressure measurement while drilling and the viscosity of mud filtrate is infinitely close to the water phase permeability under the residual oil in relative permeability experiment. Based on this, a method converting the mobility from PWD to oil phase permeability (effective permeability) is proposed. The workflow consists of the following steps: 1. Establish the relationship between water phase permeability(KW) and absolute permeability(kair) using the core relative permeability in the target oilfield. 2. Use the relationship to convert the PWD mobility(kw=λmobility *μmf)to absolute permeability. 3. Calibrate the general form of the Timur permeability model with the effective permeability derived from PWD mobility. Then the continuous absolute permeability is calculated using the calibrated Timur equation. 4. Compute the relative permeability logs to water (Krw) and oil (Kro) from known correlations. Then, calculate the continuous effective permeability to oil (Ko) as kair × Kro. 5. The well productivity is then evaluated based on the effective permeability(Ko).

For our case study, we selected 20 wells where all major pay zones are sampled evenly by PWD. We established the relationship between water phase permeability and absolute permeability(kair) from relative permeability measurements on 48 core samples. Collecting all PWD in our study field, we calibrated the field-specific Timur permeability equation and compute a continuous kair log with the calibrated model. This equation is then applied to interpreted logs in each well to produce the final absolute permeability(kair), which matches absolute permeability(kair) converted from PWD mobility very well. Calculated oil-phase relative permeability by correlation from relative permeability experiments, and then calculation effective permeability (Ko). For all study wells, the effective permeability(Ko) shows a much better correlation with the actual productivity index, compared to the absolute permeability calculated with the poroperm approach.

The novelty of our method is that we propose a calculation method of effective permeability(Ko) through PWD mobility and relative permeability test. The method takes advantage of an existing logging measurement and eliminates the need for extensive coring. The calibration leads to a more accurate effective permeability, and therefore better productivity prediction.