Lessons Learned from a Hundred Frac Packs in the Campos Basin

Abstract

Abstract This paper presents the state of the art in frac packing learned from more than a hundred operations performed in the Campos Basin, offshore Brazil. Many reservoirs in the Campos Basin are clean tertiary age sandstones, very unconsolidated, with high to ultrahigh permeability. In this scenario, in order to get low skin completions, high conductivity fractures must be created. These can only be obtained with the use of aggressive tip screen out (TSO) techniques. For these reasons, no questions remain about the superiority of frac pack when compared with conventional gravel pack. Since February 1996, when the first frac pack job was performed, many challenges have been overcome, especially those related to handling leak off in very high permeability and the correct use of rock mechanics properties for proper frac pack design. These include minifrac analysis for very low efficiency fracturing fluids, the use or not of spurt loss, how fracturing fluid efficiency is affected by prior calibration tests, horizontal stress anisotropy for unconsolidated sands, stress contrast for layered sandstone's, etc. Others lessons have been learned such as: the need for high quality fluid, high quality proppant, good proppant placement, as well as calibration tests, real time design and BHTP data collection for all jobs (from electronic gauges or from live annulus). Introduction Sand control is mandatory. This is a law in high-permeability and soft formation scenarios. In the Campos Basin this is the case. Several oil and gas reservoirs in the Campos Basin are clean, high-permeability, tertiary age and unconsolidated sandstones located at water depths ranging from 100 ft (33 m) to 6070 ft (1850 m). Up to 1996, the usual completion method for sand control was the conventional gravel pack. The usual results, very closed to the worldwide mean1, were disappointingly high skins, even using best completion practices. The first Campos Basin frac pack completion was installed at the beginning of 1996. In 1997, frac pack completions became the usual sand control method, following the development of an in-house tool and the commissioning of a new stimulation vessel, able to pump aggressive sand schedules. The first results2 showed that frac pack completions resulted in much lower skin completions, in agreement with worldwide experience. In addition, it was observed that a bad frac pack job was always better or, at least, as good as a good gravel pack in the Campos Basin soft formations. Changing from gravel pack completions to frac pack completions brought new questions and doubts, that can be roughly divided into three groups. The first is whether frac pack operations needed to follow the strict procedures applied for gravel packs, such as, high density underbalanced perforating, pickling string, cleaning risers and casing, pre-job acidizing, proppant granulometry, etc. Second, due to ultra-high permeability's, design and pumping questions were imposed: What must be the conductivity of a fracture in formations with such high permeability's, ranging from 200 mD to 10,000 mD? How to obtain such a fracture? Can available commercial simulators be used to design this kind of fracture, considering that they were developed for lower permeability treatments? Is surface pressure data accurate enough to analyze calibration tests? Is the live annulus pressure precise enough? Can it be used for calibration test analysis and to make corrective actions during the main treatment? Can a very aggressive proppant ramp be pumped or even mixed? Third, due to the complete unconsolidation of the sandstones, it is almost impossible to obtain useful cores. Rock properties, then, cannot be obtained from laboratory tests. So, how can we obtain reasonable values of the stress contrast between the reservoir rock and the constraining formations? Are these constraining formations hard enough to contain the fracture? Frac pack is a completion technique that came from merging two distinct techniques: hydraulic fracturing and gravel pack completions.