Horizontal-Well Completion and Stimulation Techniques - Review Emphasizing Low-Permeability Carbonates
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 108075, "Horizontal- Well Completion and Stimulation Techniques - A Review With Emphasis on Low-Permeability Carbonates," by Valdo Ferreira Rodrigues, SPE, and Luis Fernando Neumann, SPE, Petroleo Brasileiro S.A.; Daniel Torres, SPE, Halliburton Energy Services; Cesar Guimaraes, SPE, Schlumberger; and Ricardo Sadovski Torres, BJ Services, prepared for the 2007 SPE Latin American and Caribbean Petroleum Engineering Conference, Buenos Aires, 15–18 April.
A brief review is presented of available techniques to complete and stimulate horizontal wells, emphasizing low-permeability carbonate reservoirs. These techniques can be applied in unconventional reservoirs, particularly in tight formations. Considerations are presented to select completion and stimulation techniques. Cased and cemented horizontal wells, in addition to wells completed as open hole and with perforated/slotted liners, are addressed.

Introduction
This study focused on fracturing stimulations. It assumed that a horizontal well already is justified and that its completion and stimulation are being discussed. The macroaspects of field/area management are stressed as the completion and stimulation drivers. Key parameters for designing, evaluating, and implementing horizontal-well completion and stimulation are presented, emphasizing common failures and controversial aspects.
North Sea Carbonate Reservoirs The North Sea low-permeability chalk reservoirs are used as a reference. The Dan, Halfdan, South Arne, Valhall, and Eldfisk fields are the focus. The main characteristics of these fields include shallow water (43 to 69 m), dry completions, high volumes of original oil in place (1.6 to 2.9 billion bbl), low-permeability carbonates (0.2 to 10 md) with microfractures in the central areas (10 to 120 md), high porosities (up to 48%), soft to very soft chalks, small to medium net-pay thickness (15 to 65 m), high oil saturation (up to 97%), and light oils (approximately 36°API).
These fields are distinguished from most by overpressured soft chalks, which are subjected to a high degree of compaction under pore-pressure depletion, resulting in loss of drilling fluids, rapid production decline, well failures, and seafloor subsidence. However, the positive effects of rock compaction as reservoir-drive energy outweigh the negative effects, with the primary recovery factor as high as 30%. In general, North Sea chalks experienced an evolution from vertical and directional wells stimulated with acid treatments to multiple-fractured horizontal wells.

Campos Basin Carbonate Reservoirs
The Campos basin, offshore Brazil, was discovered in 1974, and first production was by an early-production system in August 1977. The history of low-permeability carbonate-reservoir completion and stimulation in this basin is similar to that of North Sea counterparts. In the 1980s, the completions were vertical or directional wells stimulated with massive acid fracturing or matrix acidizing. Rapid production decline was the main problem in this period. In 1989, a very large hydraulic-fracturing treatment was performed in a subsea directional well on the basis of a comprehensive study. Unfortunately, a previously unidentified fault very close to the well took most of the pumped proppant. From 1990 to 1995, stimulations were characterized by low-volume matrix treatments (up to 75 gal/ft of 15% HCl) with similar results. Beginning in 1995, with the drilling and completion (with preperforated liner) of the first horizontal well in the Quissamã formation, standard stimulation treatments comprised alternating stages of nonviscous acid, viscous acid (up to 30 gal/ft of 15% HCl), and acid with a diversion feature pumped below fracture rates, which were spotted by use of a dedicated stimulating string or coiled tubing inside the horizontal liner.