Abstract
Abstract
The Margarita-10 well is located in the Caipipendi block of southern Bolivia. This block accounts for 30% of the gas production for the entire country. An operator faced a unique challenge to reduce well construction costs for this well while drilling through a harsh Carboniferous formations sequence. The Carboniferous interval is extremely hard and abrasive with unconfined compressive strength (UCS) values ranging from 12,000 psi to 24,000 psi, averaging 20,000 psi with 32,000-psi peaks. This interval is a complex drilling environment consisting of interbedded, conglomeratic, hard, and abrasive lithologies that presents several drilling obstacles, such as formation heterogeneity and multiple geological uncertainties. These obstacles often impact drilling operations, including the bit's cutting structure being prematurely damaged that can cause nonproductive time typically related to the low rate of penetration (ROP) and poor drillsting dynamic behavior with severe levels of shocks; thus, multiple unwanted bottomhole assembly (BHA) trips.
The operator's major objective was to reduce the high well-construction costs in this environment; hence, motivating the search for a novel drill-bit cutting structure design that would produce a step change in drilling performance. To accurately determine the environmental factors that influence bit performance, a detailed formation mapping analysis was performed using geological data from nearby wells. To comprehend the causes of previous failure modes, an engineering program for rock-strength identification was used to identify the rock types and lithology characteristics, such as rock UCS, expected formation abrasion, and impact indexes. This information was used to adjust the digital rock file and better reproduce the actual drilling conditions in the Carboniferous strata virtually to develop a suitable cutting structure, predict drillstring vibrations, and recommend optimal drilling parameters to maximize the performance of the product downhole.
The operator successfully deployed the novel drill-bit cutting structure, achieving above field average results in terms of ROP and footage. The first drilling run in the Margarita-10 well drilled over 300% additional footage (compared with the offset average) at double the ROP and was pulled out of the hole (POOH) with an excellent dull condition. The drilling second run was equally successful, with 332% additional footage drilled and 45% higher ROP compared with the offset average. These results reduced the rig time by 20 days, resulting in significant well construction costs savings for the operator. Moreover, considering 2.6 kg of CO2 per liter of diesel burned daily resulted in allowing more than 600 t of CO2 emission reduction.