The Veracruz basin, located in eastern Mexico, has sometimes been very difficult to evaluate with openhole wireline tools because over-pressured gas formations present a problem when trying to reach a balanced borehole condition. One of the fields was planned for development using nearly horizontal wells to maximize production, but acquiring openhole logs in these wells has proven to be difficult.
Well A-21 is the first highly horizontal well to be completed using only LWD resistivity data in the nearly horizontal section. To assist in the evaluation after the well was completely drilled and cased, a pulsed neutron tool was logged across the entire well using a tractor device to reach the total depth of the well. An evaluation model for the A field was then developed using the CHI (cased hole interpretation)modeling program, by using the openhole resistivity and porosity data acquired in the vertical A-1 well, combined with the cased hole pulsed neutron data acquired across the same interval. Pseudo resistivity and porosity logs were then created for the A-21 well using only the pulsed neutron data acquired across the nearly horizontal section of the well, based on the Chi Model developed for the field. The pseudo resistivity was then compared to the LWD resistivity data acquired in the A-21 well. Next, the interpretation was completed by defining the optimum perforating interval for the reservoir conditions and the mechanical condition of the well. After evaluating the interpretation, the On-Balance perforating technique using coiled tubing was decided upon as the optimum technique to perforate the A-21 well, to minimize reservoir damage. This paper will present the procedures used to evaluate and complete the A-21 and the A-31 wells, as well as a comparison of the CHI Modeling pseudo resistivity with the LWD resistivity measurement.
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. The integration of the data obtained, along with applied reservoir geomechanics, for perforating design and production planning is shown to be a valid alternative to maximize production, and to prevent sanding and completion problems while reducing costs.
Introduction
Highly deviated wells have traditionally required the use of either logging-while-drilling (LWD) or tool pusher techniques to acquire the information needed to perform formation volumetric evaluation. This evaluation is used to determine the best intervals to be completed. A new technique is currently available that can be used in development fields to optimize drilling and completion cost while minimizing the risk of getting stuck with a set of LWD tools. Sticking LWD tools downhole can lead to a complicated recovery of the drilling string. In addition, because they contain chemical radioactive sources, LWD porosity tools left in the well present further complications.
The innovative technique presented in this paper includes an application of pulsed neutron log data to acquire pseudo openhole data after the completion of the well. This acquisition can even be accomplished without a drilling rig, which further minimizes both the risk and the cost of logging the openhole section. This paper presents the theory behind the technique and the process required to reach a result. It also includes techniques currently used to acquire the information in cased hole, such as wireline logging and perforating using either coiled tubing or the wireline tractor tool.
