An accurate knowledge of vertical permeability (kv) in a thick reservoir is critically important to the design of a successful reservoir sweep development. However, the in-situ vertical permeability is very difficult to ascertain and may not be the same as those measured from core analyses. In this paper the usefulness of understanding ‘abnormal’ RFT data from a producing field is presented, together with a proposed methodology to estimate in-situ vertical permeability through matching RFT data under dynamic conditions.
An ‘abnormal’ vertical pressure gradient (twice the expected value) was detected by RFT within a 140 feet thick limestone reservoir in close proximity to an active producer. Attempts were made to match the data using a fine grid 3D simulation sector model whilst honouring conventional core analysis data (kv/kh ratio of 0.2 to 0.7). It was found impossible to match the gradient with this relatively high kv/kh ratio, even with only the upper perforations on production (indicated by historical PLT logs). Several different assumptions were required to achieve an acceptable pressure distribution match. One case assumed Stylolite layers had very low permeability. In another, uniformly lower vertical permeability for entire layers was necessary. Although a unique solution was not possible, the results strongly suggested that vertical permeability might be lower than previously understood.
Due to the impact of reduced vertical permeability upon future field development, a specialized test to reliably measure in-situ vertical permeability was designed via 3D simulation. The test monitors a pressure wave generated by a partial penetration well at an offset, fully penetrating, well via RFT measurements. The expected pressure responses for such a test are presented.