Selection of Foam Flushes and Foamed Cement Proves Effective for Remedial Operations of Low-Pressured Formations—Case Histories

Abstract

Remedial cementing operations and zone abandonment are difficult to accomplish when low-pressure formations possess high feed rates at low, or even vacuum, surface pressures. Such is the case with several heavy-oil wells recently abandoned in Western Canada. These wells and their respective formations average an economic life of approximately ten years, during which a large amount of formation sand is produced. Traditionally, whenever exposed to hydrostatic pressure, these zones exhibit a vacuum at very large feedrates. Scorecard results of conventional cement squeezes required an average of 2.6 attempts to obtain a positive squeeze pressure. Many conventional cementing abandonment attempts were unsuccessful, resulting in the placement of a casing patch across the open perforations. The mechanical seal was effective in sealing the wellbore, which allowed exploitation of deeper zones. However, this seal reduced the casing internal diameter and can limit further production operations. Also, this mechanical seal may not serve as a final abandonment under certain government regulations. The solution to this challenge was developed by tailoring a treatment pumping schedule that incorporates the use of foamed nonreactive, reactive spacers and foamed cement blends. This new treatment design was used on several wells in early 2002. On most of the wells, positive squeeze pressure was obtained on the first attempt and saved approximately CAD $15,000 per well. The improved process is now being planned for two adjacent fields. This paper details the well challenges and results found by a Canadian operator to successfully abandon low-pressure, heavy-oil producing zones. The solution developed for the case history wells is also presented. Introduction When the economic limit of a hydrocarbon producing well is reached, the well or zone should be plugged and abandoned. Traditional abandonment and sealing techniques include cement squeezes, gel squeezes, bridge plugs, patches, scab liners, and straddle packers.1,2 Cement and gel technologies are typically used for behind-casing repair, and mechanical workover options are usually confined to sealing the casing. For example, a successful technique in completing the abandonment process incorporates the use of mechanical means such as casing patches, casing clad, and/or bridge plugs. Once the mechanical barrier is set above or across the target zone, the wellbore can be circulated with a full hydrostatic column and the well abandoned. However, some government regulatory agencies do not accept mechanical seals as a final abandonment of the well. Typically, to meet regulatory guidelines, the operators must place cement slurry across the open or perforated section. Unfortunately, conventional cement slurries, which have densities from1680 kg/m3 to 2040 kg/m3 (14 to 17 lb/gal) will quickly build hydrostatic pressure in the casing relative to the target formation. In the case of abandoning low-pressure zones, the density of the cement column could fracture the target formation. The hydrostatic pressure of the cement column is high enough to initiate fluid flow into the formation, and the entire perforated interval may not be covered with cement. This results in some of the perforations being sealed while other adjacent perforations remain open. As mentioned earlier, cement technology is commonly a standard technique for zone abandonment. Cements provide a strong, near-wellbore block of gas and fluid production and are able to fill perforation tunnels, channels behind pipe, and washout zones behind pipe. Cement that has adequate strength development and other properties can be designed to withstand future fracture treatments and acid stimulations. However, cement has limitations, such as difficulty penetrating deeply porous rock of a potential gas source or microchannels that develop from cement sheath cracking or poor mud displacement (Figs. 1–3),3–9 resulting in interzone fluid communication.

Foamed Cement Applications in Oil Industry Based on Field Experience: A Comprehensive Review

“…The required number of treatments needed to establish the mechanical seal was reduced. 68 Five water-injection wells in different fields in Oman (two in North and three in South Oman) were experiencing bad zonal isolation problems across fractured formations that led to corrosion of the casing strings. Cementing of tieback casings that run across the corroded ones using foam cement results in improvement in the zonal isolation across the corroded casing and fractured formations.…”

“…In early 2002, pumping of foamed spacers and cement blends on several wells to abandon low-pressure heavy oil sands in Western Canada resulted in obtaining a positive squeeze pressure in many cases from the first attempt where many of the conventional cementing were unsuccessful. The required number of treatments needed to establish the mechanical seal was reduced …”