NMR as Standard Porosity Tool in the San Jorge Basin: Log Responses and Applications
Authors: Solanet F. , Khatchikian A. , Breda E. 
Published: December 2007

This paper describes our experience with Nuclear Magnetic Resonance (NMR) logs since their introduction in the San Jorge basin in June 1995. To date, more than 200 logs have been run by Western Atlas and Schlumberger using Numar's MRIL and CMR tools respectively and more than 2000 zones have been perforated and tested. This has allowed us to characterize the response of the tools in shaly and tuffaceous sands, compare the response of the Numar and Schlumberger tools amongst themselves and with cores, detect the limitations of present tool designs and relate NMR pore-size distribution and permeability to oil production.

Introduction

The bulk of YPF's development drilling in 1995 and 1996 took place in the San Jorge basin in Southern Argentina, where more than 1600 wells were drilled in 1995-1996. Production and reserves per well in this basin are relatively low, and therefore it is essential to keep costs at a minimum while optimizing production per well. To this end, several new drilling, logging and completion technologies were tested to evaluate their contribution in increasing production and decreasing the overall development cost.

One of the main problems during the completion of the well is the high number of dry zones. In some cases dry zones can be predicted by their low porosity, or no SP development, or no Microlog separation. But in many cases, zones that look "good" on logs are dry or have very little oil production. Since the lack of production can be due either to low permeability or formation damage, the effect of stimulating these zones is unpredictable: production can either stay the same, increase a little or increase by a factor of 10 or more. Under these conditions, a measurement with the potential to discriminate between low permeability and damaged zones can save a considerable amount of money in perforating and stimulation.

In June 1995 Western Atlas run the first NMR log using Numar's MRIL tool. The same month Schlumberger ran their first NMR with the CMR tool. Following a brief evaluation period, we found that the NMR-derived permeability was a valuable information to help decide which zones should be perforated and/or stimulated. To date, more than 200 NMR logs have been run by Western Atlas and Schlumberger with MRIL and CMR respectively. The present rate is about 20 NMR logs per month, i.e., about one well in three is logged with MRIL or CMR.

MRIL vs. CMR

Comparisons between the Numar and Schlumberger tools were made in two wells. The agreement of NMR porosity and permeability from the two logs is generally good, as shown on the example of Fig. 1, which corresponds to the first well. The differences can be attributed to the different vertical resolution of the tools and the effect of rugosity or loss of pad contact on the CMR.

NMR porosity

Comparison with conventional porosity logs - The most striking observation on the first NMR logs was that NMR porosity was systematically lower than density porosity. In this basin it is known that the density log may not be adequate for porosity evaluation because of the very low density of some tuffaceous sands with a high zeolite content. In this case it is better to derive porosity from the sonic log, which is less lithology-dependent. However, in areas of shallow reservoirs (less than 1200 meters) the density log is preferred as the sonic may overestimate porosity because of lack of compaction. A neutron log is also run when there is a possibility that some of the sands be gas-bearing. P. 183