For 15 years, HPHT wells have been cemented in northern Italy, under challenging and often extreme conditions. The need to maintain zonal isolation and casing support, whilst at the same time being able to tolerate massive changes in both temperature and pressure, has led to the development of some highly innovative cement slurry systems.
Chief among the innovations introduced has been the use of manganese tetraoxide as a weighting agent, instead of the traditional hematite. Pre-blended into the mix water, rather than dry blended into the cement powder, this additive has had a massive impact on slurry quality and is now employed as "best practice" throughout the region.
Cement and silica flour remain the most important materials for insuring good and consistent properties in the slurry. Consistent quality of these materials is vital - something that has not always been maintained in the past, resulting in random slurry properties and highly intensive testing. Improved management of the source of these additives has improved the consistency of the test results and reliability of the slurries.
Slurry testing conditions should be customised to be as close as possible to the actual well conditions. In particular, selection of the appropriate consistometer test temperature ramp has proved to be particularly critical.
This paper will highlight the difficulties encountered when designing, testing and executing HPHT cementing jobs. Several cases will be examined in order to illustrate the problems encountered and their solutions.
The High Pressure and High Temperature (HPHT) wells are classified as having extreme conditions in which BHT (Bottomhole Temperature) exceeds 121 °C (250 °F) and pressure gradient more than 2.23 bar/10m.
HPHT cement systems require special design attention, modified testing procedures and special products. To design an HPHT slurry, the engineer must collect fundamental well parameters, such as accurate well data (depth, deviation, hole size, logging data, fluid lost and any other problems during drilling). The engineer must also predict Bottomhole Circulating Temperature (BHCT) using validated temperature software. Job design will then require a job design simulator, specific laboratory testing to meet API guidelines, a good mud removal evaluation, Quality Control (QC) of materials on site, and a contingency plan in case conditions on location are not as expected. This paper presents 15 years experience of cementing HPHT wells and all the difficulties encountered in designing and executing such operations.
The Po Valley area (Northern Italy) is home to a number of HPHT wells, such as the two case histories presented in this paper.