Using Downhole Pressure Gauges in Hostile Deep Wells, Villafortuna-Trecate Field
Authors: Botto Giovanni , De Ghetto Giambattista 
Published: December 2007

Summary
This paper summarizes extensive, pioneering experience with the first 5-year well-testing project in the hostile ultradeep wells of the Villafortuna-Trecate field. Performance of the various downhole high-temperature, high-pressure (HTHP) gauges used is investigated, and the relevant technology is compared throughout the field history. Awareness of the quality of data acquired and the role played by redundancy (use of more gauges simultaneously) are identified as key points in planning successful future HTHP operations.

Introduction
Hydrocarbon E?P is moving toward more hostile, deeper, and hotter reservoirs. The acquisition of reliable downhole data for reservoir management in such harsh conditions is a challenging task that needs dedicated and rugged, but sophisticated, pressure gauges.
Villafortuna-Trecate is a producing oil field in the Po Valley of northern Italy (Fig. 1). The first hydrocarbon discoveries were made in 1984, and subsequent exploration led to the definition of two distinct reservoirs: the upper level Conchodon dolomite and the lower level Monte San Giorgio dolomite. Table 1 shows the Villafortuna-Trecate reservoir characteristics.
Because of the simultaneous presence of CO2 and H2S, the wells are completed with corrosion-resistant-alloy (CRA) tubular goods, mainly 3 1/2- and 2 7/8-in.-OD single tapered string (Fig. 2). A major concern is asphaltene plugging downhole, which causes a reduction in production that must be restored by solvent treatments. Matrix acidizing is another standard treatment for skin removal. Finally, field location within the protected area of Ticino Natural Park requires careful evaluation of the environmental impact of every operation.
Because of the complexity of the reservoir (presence of natural fractures and faults), the determination of significant reservoir and production parameters (e.g., skin factor, permeability pay thickness, and PI) requires long-term (a few days) operations after the completion of each well (Fig. 3). To evaluate the effectiveness of the remedial jobs, additional short-term operations generally follow asphaltene removal and matrix stimulation.

Field History
This analysis of the well-testing operations [well-testing operation or operation: downhole measurement(s) performed with an electronic temporary gauge(s) supplied by a service company(ies), whose goal is the acquisition of reliable pressure and temperature data] covers the 5 years from mid-1987 to mid-1992. The first well, Well VF-1, actually was tested in Dec. 1984, but neither the electronic nor the mechanical gauge was able to collect downhole data; extensive operations date from May 1987. The collected information is organized in tables to give a clearer picture of the results.
Table 2 summarizes the gauge configurations used. Until 1991, most jobs were performed with a single gauge operating in "surface-readout" mode; a few were performed with two and sometimes three tandem gauges. By the beginning of 1992, owing to the availability of more reliable batteries and memory sections, service companies often provided "memory" gauges, mainly to perform short-term operations. Each well was tested at least once after completion.
Table 3 shows the extent of the overall gauge failures, 24 out of 62 operations, with a consistent level of failures throughout the years; 1992 data are for the first 6 months. Generally, failures can be divided into gauge and cable failures; cable failures caused some gauges to fall into the hole and be lost. Because the objective of this paper is to examine gauge performance, we will discuss only the gauge failures.
Table 4 focuses on the four service companies involved in the operations (Companies A through D). In addition to the general item "gauge failures," we have included "multiple failures" (gauge failures involving more than one gauge) and "bad or no data acquisition" (gauge failures that result in no acquisition of reliable pressure and temperature data) to provide a better explanation of the types of failures that occurred. Companies C and D performed only short-term jobs that use memory gauges since 1991, jointly &lt;12% of the total (well-testing job or job: an operation referred to a service company; the number of jobs is equal to or greater than the number of operations because more than one service company could be simultaneously involved in one operation), while Companies A and B performed the statistically significant bulk of the jobs, 32% and 57%, respectively. Note that, in absolute terms, (e.g., gauge failures) Company B performed better than Company A (31% and 55% of the relevant jobs, respectively); the situation is reversed for the most critical multiple failures (55% and 18% of the relevant gauge failures, respectively) and bad or no data acquisition (55% and 36% of the relevant gauge failures, respectively).
Tables 5 and 6 show the performance of the gauges supplied by Companies A and B throughout the years.

HTHP Gauge Technology
Service companies were asked the reasons for such unsatisfactory performance. Their answers provided some information on HTHP gauge technology. No single technological approach to HTHP gauges exists. Among the four service companies (three are also gauge manufacturers) involved in the Villafortuna-Trecate well testing, Company A had always proposed strain gauges, Company B had always proposed capacitance gauges, and Companies C and D had always proposed quartz gauges. We doubt these positions are decided by commercial policy, but examination of what is entailed in the choice of one type of gauge is useful. Consider that a specific gauge model is made up of two distinct parts: the "transducer" (i.e., the part of the gauge directly in contact with the environment) and the "electronics" (i.e., the part of the gauge devoted to processing the output signal of the transducer). Assume that each HTHP gauge contains a certain amount of "technological sophistication" to be shared between the transducer and the electronics. In a strain gauge, the transducer is tough, but it provides an analog output signal that has to be processed, if not digitized, by the gauge electronics downhole. The need to operate with a discontinuous amplitude signal to minimize the noise is the reason for this processing. Such processing requires the presence of certain electronics whose reliability in an HTHP environment is doubtful. In contrast, gap capacitance and quartz transducers provide a frequency signal as output, which requires less electronic processing; here, the critical item would be the weakness of the transducer. Table 7 summarizes this simplified HTHP gauge classification.