Application of CHI Modelingsm Using Pulsed Neutron to Create Pseudo-Open Hole Logs

Abstract The Burgos Basin, located in Northeastern Mexico, has sometimes been very difficult to evaluate with openhole wireline tools because over-pressured formations present a problem when trying to reach a balanced borehole condition. This paper will discuss three problem wells, presenting both CHI Modeling results used to solve the problems as well as an alternative method of obtaining resistivity behind casing. Case History #1 looks at the Fundador Field which has been a particular problem in this area, presenting both drilling and logging difficulties. Because of borehole problems associated with over-pressured formations encountered in this well, it was not possible to acquire openhole wireline logs across the intermediate drilling zone. After the well was completely drilled and cased, a pulsed neutron tool was run across the entire well to assist in the evaluation. By using the resistivity data acquired in the previous section of openhole along with the pulsed neutron data logged in cased hole across the same interval, the missing resistivity data was predicted over the entire intermediate zone. The pseudo resistivity log was created using only the pulsed neutron data obtained across the intermediate zone, processed with the CHI (Cased Hole Interpretation) Modeling program. The final resistivity is also compared to a second method of obtaining resistivity behind casing used by another service company. Case History #2 uses CHI Modeling to create pseudo openhole logs that were subsequently used as inputs to a volumetric evaluation. The CHI Modeling results are compared to an alternate method of obtaining resistivity behind casing. Testing results are documented to confirm the validity of both the CHI Modeling and the volumetric evaluation. Case History #3 documents where the CHI Modeling technique was successfully applied through two strings of casing where density data was not available. A comparison to the original openhole logs is presented confirming that this technique is a valid option when additional openhole information is required in a partially logged well. This paper will discuss the procedures used to evaluate three case histories using CHI Modeling, and present two comparisons with a second cased hole resistivity measurement. Production data is also presented to validate this technique. The use of pulsed neutron logs to acquire pseudo openhole data is shown to be a valid alternative when drilling conditions do not permit normal data acquisition in openhole; or when additional openhole information is required after the well has been cased.