Breakthrough Polymer Water-Shutoff System Shows Promise for Carbonate Ghawar Field
A new water-shutoff polymer system has been developed for carbonate formations and shows great stability.
Excessive water production from hydrocarbon-producing wells can adversely affect the economic life of the well. The major challenge for water control in carbonate reservoirs is polymer bonding to the rock surface. Most commercial products are designed for sandstone formations, and most polymers will not strongly adsorb to carbonate reservoirs. A new water-shutoff polymer system has been developed for carbonate formations and shows great stability.
The objective of the project was to develop additives to be used as a smart sealant that can be used to control unwanted water production. Treatment of water associated with hydrocarbon production is a key goal because the production of salt water has resulted in serious environmental issues. Excess water production makes a well unproductive and economically inefficient, leading to early abandonment of wells and reduction in hydrocarbon production.
Reservoir heterogeneity is the single most important cause of low oil recovery and early excess water production. Gel treatments have been used extensively in field applications to improve oil recovery and suppress water production. Gel treatments at injection wells to plug water thief zones are a proven cost-effective method to improve sweep efficiency. In addition, gel treatments reduce excess water production during hydrocarbon production.
The initial focus of the project was to develop materials that impart relative permeability modification in naturally fractured carbonate reservoirs with permeability from 1 to 3 darcies in the Ghawar Field, which is a heterogeneous carbonate.
Chemical functionality that can bond strongly to the target carbonate reservoirs is an extremely important component of the development of the smart sealant. Methodologies to provide strong adhesion to carbonate surfaces, and hence to formations, have been developed using silicon-containing molecules. In addition, the additives developed have shown promise on both water- and oil-wet surfaces.
The list of chemicals that could be assessed as additives to bond to the carbonate surface was long, so the team developed a laboratory-based screen to test adhesion rapidly. For the first screens, calcium carbonate powder was a good model because it is readily available and is a major constituent of carbonate reservoirs. The experiments involved taking accurately weighed samples of calcium carbonate and exposing them to different target additives that could potentially bond to the surface. Subsequently, the samples were washed extensively with water to remove any materials that had not chemically adhered to the surface, were dried under reduced pressure, and finally were reweighed. Any significant weight increases were indicative of bonding.
The method was used to identify inorganic silicate-based functionality that displayed good adsorption to the model reservoir material, and a decision was made to test this adsorption system further.
Arab-D reservoirs typically are neutral to oil-wet; therefore, the screening experiments were carried out with calcium carbonate that was exposed to an oil phase. The adsorption system was tested on a water-wet calcium carbonate sample and a calcium carbonate sample that was exposed to oil. For the samples exposed to oil, the calcium carbonate powder was mixed into an oil phase and left for 48 hours. The calcium carbonate was removed by filtration, the sample was dried, and the adsorption tests were conducted. The amount of material adsorbed was determined gravimetrically, and both samples gave the same uptake, which shows that, in this case, wettability did not have a large effect.
Essentially three interfaces need to be considered: adsorption functionality to reservoir rock, adsorption functionality to the polymeric system, and polymer-to-polymer adsorption functionality. The system can be crosslinked, which means that the third interface is already satisfied.
The silicate-based adsorption system is capable of bonding to the calcium carbonate surface (the first interface) according to observation, so the next logical stage was to demonstrate that a polymer that is capable of providing a water-shutoff effect could be anchored to this novel adsorption system (the second interface). Tests were performed with the adsorption system but, this time, in the presence of a polymer that was capable of chemically bonding to the adsorption system. When this was attempted, higher mass gains were observed than with just the adsorption system, which can be attributed to the polymers being designed to covalently bond to the surface. Control experiments with polymers that were incapable of bonding to the adsorption system did not show any increased adsorption. In addition, the polymer alone did not show any adsorption to the model carbonate and no weight increase was observed.
In the cases in which polymers were included, the wettability of the calcium carbonate did have an effect on the gravimetric uptake.
Two types of polymer solutions were used in this test. The first one was the smart sealant. The other polymer was organically crosslinked polymer (OCP). A buffer system was used to maintain neutral pH. Brine solution was prepared with 2 wt% potassium chloride (KCl) with viscosity of 0.3214 cp at 200°F. Formation water collected from the field was used to prepare the polymers. Fractured carbonate-reservoir core plugs were used.
Thermal Stability. The viscosity of the smart sealant and the OCP fluids, prepared at different concentrations and with different brines, was measured. Then, samples were stored in sealed tubes in a thermostat-controlled oil bath. Test tubes were visually inspected periodically at temperature without cooling for any precipitation. Viscosity of the samples was also measured before and after the thermal test.
Rheological Measurements. Rheological measurements were conducted on fluids made with smart sealant and OCP. Effects of polymer concentration, additives, and high temperatures on viscosity were studied.
Coreflood Tests. Coreflood tests were performed to determine smart sealant and OCP efficiency and stability. The flow sequence of fluid injection was brine, polymer, brine.
Environmental Surface Electron Microscopy (ESEM) and Energy Dispersive (EDS) X-Ray. ESEM and EDS X-ray analysis was conducted to characterize fractured-core-plug samples before and after the smart-sealant treatment.
Thermal Stability. Compatibility and stability tests of the smart sealant were conducted at reservoir temperature. The polymer solution did not show any precipitation with brines for 1 month. However, polymer became more viscous and formed gel at reservoir temperatures after 24 hours, which will ensure water conformance downhole. The results for OCP showed compatibility at different concentrations with brine. Solutions were placed in an oven for 1 month; however, no gel or viscous materials were formed at high temperature (149°F).
Smart-Sealant Viscosity. The smart sealant showed a decrease in viscosity with an increase of shear rate (i.e., a shear-thinning behavior) at room temperature. The low viscosity at surface conditions (low temperature) will enhance the pumping rate during field applications. The viscosity of the smart sealant increased at reservoir conditions, however, and exhibits shear-thickening behavior at high temperatures. This can suggest that crosslinking is taking place with the smart sealant at reservoir conditions, which will enhance water shutoff. As the smart-sealant viscosity increases at high temperature, it will help form gel and seal the treated zone.
OCP Viscosity. Viscosity of OCP did not exhibit significant thinning behavior with the increase of shear rate; however, the viscosity is relatively low at all rates. Salinity did not affect OCP viscosity significantly; however, the viscosity was generally low with all measured salt concentrations. Viscosity was measured at 4, 7, and 10 wt% KCl, with no significant effect on gel viscosity. The results can indicate also that there is no crosslinking taking place.
Coreflood Tests. Coreflood experiments were conducted to evaluate the tendency of OCP and smart sealant to shut off water production in carbonate cores and the stability of the results. The plan was set to treat fractured and high-permeability carbonate rock samples at 140°F.
Fractured Core Sample. A carbonate core-plug sample was manually fractured to represent actual downhole fractures that might have caused the severe production of brine. Results of the coreflood test of the fractured carbonate core sample with the smart sealant showed that pressure drop was increased from almost zero to 6–7 psi, with great stability. Smart sealant showed stability with 400 pore volumes of brine injection after treatment. The results confirm that the smart sealant adsorbs to carbonate rock samples better than OCP does.
High-Permeability Carbonate Plug. Brine injection to a high-permeability plug sample showed that smart sealant was extremely effective for shutting off production with great polymer stability at 140°F. Smart sealant also was tested with a high-permeability carbonate rock sample. An initial-permeability test of the high-permeability plug sample showed permeability of 4,000 md before the polymer injection. Then, 10 pore volumes of smart sealant was injected into the rock sample at 1 cm3/min and allowed to soak for 1 day. Results showed that the smart sealant reduced brine permeability from 4,000 to 40 md with great stability.
Microscopic characterizations were conducted for before and after treatment of carbonate core samples with smart sealant. Two core samples were tested, a high-permeability core and a fractured core. The aim of the test was to investigate adsorption and stability of the smart-sealant polymer on the rock surface after flooding with 400 pore volumes of formation brine.
ESEM backscattered and secondary electron images were acquired from different parts of the samples at different magnifications.
High-Permeability Core. The core was tested before and after treatment with the smart sealant. The obtained morphological images showed that the untreated core-plug sample contained largely unblocked sub- and micropores.
The ESEM and EDS characterization of the chemically treated core plug indicated the formation of dendrite-like structures on the surface of the samples (Fig. 1 above). Furthermore, EDS measurements showed the presence of a silicon compound. Silicon is the adsorption system used to bond smart sealant to the rock surface, so the results confirm adsorption and stability of the polymer, which blocked microfractures.
Fractured Core. The cross-sectional area of the fractured carbonate plug sample was tested before and after treatment with the smart sealant. ESEM and EDS results revealed the presence of silicon and carbon-rich compounds filling fractures, as depicted in Fig. 1. This confirms that the smart-sealant treatment of the core plug resulted in some of the used polymer product blocking fractures and pores. This finding confirms adsorption and stability of the smart-sealant polymer on the rock surface, blocking fractures and reducing permeability of fractured core samples.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 183558, “A Breakthrough Water-Shutoff System for Super-K Zones in Carbonate Ghawar Field: Adsorption and Polymer System,” by Ayman R. Al-Nakhli, SPE, Mohammed Bataweel, SPE, Ayman Almohsin, SPE, and Hameed Al-Badairy, Saudi Aramco, prepared for the 2016 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 7–10 November. The paper has not been peer reviewed.