Development and Evaluation of Gel-based …

1 SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2016 ABSTRACTThe extreme heterogeneity of carbonate reservoirs, featuring fracture corridors and super-permeability thief zones, chal-lenges the efficient sweep of oil in both secondary and tertiary recovery operations. In such reservoirs, conformance control is crucial to ensure injected water and any enhanced oil recovery (EOR) chemicals contact the remaining oil in an optimal man-ner with minimal throughput. Gel-based conformance control has been successfully applied in both sandstone and carbonate reservoirs. In-depth conformance control in high temperature reservoirs is still a challenge, due to severe gel syneresis and the associated significant reduction in gelation time. In this work, a laboratory study was conducted to evaluate a polyacrylamide/chromium gel system for application in a high salinity and high temperature carbonate reservoir.

2 The gel formulation was evaluated using different lab experiments, including tests of gelation time, gel strength and long-term sta-bility. Gelation time was determined by bottle tests and viscos-ity measurements. Gel strength was estimated with rheological measurements. Long-term stability was evaluated both in bulk and in situ using core samples. In the latter case using cores, a single-phase displacement experiment was performed to see if the selected gel system could effectively block high permeability zones. Multiphase displacement experiments were then con-ducted to assess the effectiveness of the gel treatment in oil re-covery improvement. In these tests, the gel solution was injected into specially prepared heterogeneous carbonate core samples, where high permeability channels had been created by drilling holes through two of the core plugs, which were brought to-gether to constitute a composite core single-phase displacement experiments showed a signif-icant reduction in brine permeability after gel treatment.

3 Even when the bulk gel volume was reduced by 50%, the original brine permeability fell by more than 80%. The multiphase displacement experiments demonstrated significant incremen-tal recoveries of oil post-gel treatment. For example, a sur-factant polymer (SP) flood conducted after the gel treatment resulted in an ultimate oil recovery of 83% original oil in core (OOIC), while without the gel treatment, the SP flood attained an ultimate oil recovery of 67% OOIC. Further results in this study demonstrated the potential of the studied gel system for improving oil recovery and its favorable impact on sweep efficiency for both water and chemical flooding applications in high temperature and high salinity oil recovery in waterflooding and enhanced oil recovery (EOR) processes depends on both microscopic dis-placement and volumetric sweep efficiencies.

4 In chemical EOR processes, surfactants are used to improve the microscopic displacement efficiency, mainly through a reduction in the oil-water interfacial tension. For extremely heterogeneous carbonate reservoirs, improving the sweep efficiency is also critical, but fracture corridors and super-permeability thief zones present challenges in both secondary and tertiary recov-ery operations. Moreover, when chemical flooding is applied to such heterogeneous reservoirs, the injected chemicals tend to flow along the higher permeability channels, where the flow resistance is much lower. Although polymers are often applied using post-surfactant injection to improve sweep efficiency by reducing the mobility ratio, and so improving flood front stability, their ability to overcome extreme heterogeneities is limited.

5 Therefore, to ensure EOR chemicals efficiently con-tact the remaining oil with minimal throughput, conformance control is crucial. Both mechanical and chemical conformance methods have been widely used for water shut off and profile modification across producers and injectors, respectively. Among chemical conformance methods, polymer gels are widely used due to their low cost and ease of application at well sites. Polymer gel technology for conformance control has had worldwide success. When a gelant solution consisting of a water-soluble polymer and crosslinker(s) is injected, a solid-like gel system with a 3D network structure forms after a certain time in response to elevated reservoir temperatures. As a result, any subsequent injection water is diverted to the unswept and/or less well swept regions.

6 Applying gelants via in-jectors is called profile modification. (In water shut-off applica-tions, a gelant is injected across a production well to block and/or reduce the unwanted excess water and/or gas production.) Many types of gelants are available. Polyacrylamides with varied degrees of hydrolysis and molecular weights are the most commonly used. Gels can form due to chemical bonding Development and Evaluation of Gel-based Conformance Control for a High Salinity and High Temperature CarbonateAuthors: Dr. Jinxun Wang, Dr. Abdulkarim M. Al-Sofi and Abdullah M. Al-BoqmiFALL 2016 SAUDI ARAMCO JOURNAL OF TECHNOLOGY between the negatively charged carboxylate groups of the polymer and the multivalent cations of a crosslinker, such as trivalent chromium (Cr(III)).

7 Sydansk and Southwell (1998)1 reviewed more than 12 years of experience in developing and applying the widely used Cr(III) carboxylate/acrylamide polymer gel technology. Other studies have investigated gel production through the formation of covalent bonds be-tween organic crosslinkers and the functional groups of the polymer2-6. Vossoughi (2000)7 reviewed a number of profile modification gel systems, including a new type of bio-polymer gel system free of crosslinkers that initiates gelation by pH reduction. A more recent review of polymer gel systems was presented by El-Karsani et al. (2014)8. Han et al. (2014)9 re-viewed Gel-based in-depth fluid diversion technologies, includ-ing weak gels, sequential injection for in situ gels, colloidal dispersion gels, microgels and preformed particle gels.

8 The selection of a given gelant formulation strongly de-pends on the specific conformance problem and reservoir conditions. For a successful field application, adequate gela-tion time is required to pump the gelant solution to the target treatment region. The polymer and crosslinker types and their concentrations, as well as the reservoir temperature, salinity and pH, all have influence on the gelation time. Because tem-perature has a significant impact higher temperatures result in shorter gelation times retarding agents are often used to lengthen the gelation time in high temperature reservoirs. High temperatures also have a significant impact on syneresis. Water-based gels have high water content trapped in their 3D network structure7.

9 Under some conditions and/or with time, these gels can undergo syneresis where water is expelled from the gel structure, which then shrinks. This phenomenon can negatively affect the gel s long-term stability, especially for treatments and/or placements across fractured zones. Gel sy-neresis is more severe at higher temperatures and under higher hardness conditions. Moradi-Araghi (2000)10 reviewed gelling systems effective in high temperature, high salinity, and high hardness reservoirs. In this article, a laboratory study was conducted on a poly-mer and Cr(III) gel system intended for application in a high salinity and high temperature carbonate reservoir. Gel prop-erties were evaluated by measuring gelation time, gel strength and long-term stability.

10 Then oil recovery improvement after treatment using the gel system was demonstrated through coreflooding tests conducted at reservoir conditions on spe-cially prepared heterogeneous carbonate core samples. MATERIALSOilA dead crude oil sample was used in this study. At room temperature, oil density and viscosity were g/cm3 and centipoise (cP), respectively. At 95 C, the oil density and viscosity were g/cm3 and cP, respectively. The oil was filtrated through a 5 mm brines were prepared and used in this study. Details on the compositions and properties of the synthetic brines are listed in Table 1. All brines were filtered through a mi-cron filter and sulfonated polyacrylamide a copolymer of acrylamide and acrylamide tert-butyl sulfonate with a sulfonation de-gree of about 2% was used to form the gels in this study.