Fluid-Rock interactions were evaluated using formation rocks and fluids from Carito Field at East of Venezuela, in order to establish better criteria for the design of more efficient cement spacers. Rock and formation fluids were analyzed before and after exposure to overbalanced drilling and cementing filtrates through formation cores. Rock changes were evaluated by X-ray diffraction, SEM, permeability and wettability, whereas fluids were analyzed by fluid and gas chromatography, GC - MS, NMR and IR. Results indicate that 100% oil based mud filtrate produces a large permeability reduction. Spacer A showed greater mud removal efficiency than spacer B, whereas the cement slurry filtrate generates lighter formation permeability reduction than the oil-based mud, after either spacer exposure. This can be explained due to insoluble salts' precipitation when the cement filtrate is in contact to the residual formation water. The molecular analysis of crude samples indicates that the absence of heavy molecules from hopane compounds in the crude saturated fraction could be related to adsorption of these compounds on the rock surface, which is partially oil wettable. In addition, absence of C15 - C20 molecules from crude oil is related to molecular distribution in the gasoil from mud and spacers evaluated. The results of these tests indicate interaction between crude oil and rock composition with drilling and cementing filtrates. Overall results of this study will allow achieving a more efficient approach for design of spacer fluids. Introduction Formation damage during drilling and cementing operations have became important in the oil industry lately because of severe chemical and mineralogical changes observed to the rock and fluids from reservoirs during these processes. Oil based mud is a technology relatively new, thus few studies (Refs. 1 through 3) on the formation damage these fluids cause have been published only recently. Physicochemical equilibrium in the reservoir is altered when rock formation is exposed to drilling and cementing fluid filtrates, especially on rock permeability, wettability, water saturation and crude-oil properties. These alterations can directly cause impairment and loss in reservoir productivity (Refs. 4 through 8). Cementing, as drilling, is an important process in all phases of the well development. This process also may require oil-based fluids as washer and spacers that contain different kinds of surfactants and polymers useful to improve cement adherence to the casing and formation, when oil based mud are used. Cement slurries also have high calcium concentration and pH, which results in insoluble salts precipitation in the porous media that reduce considerably the rock permeability. Other additives are commonly added to cement formulations in order to disperse cement particles, modify the setting time under temperature and pressure conditions, control fluid losses from the cement slurry during and after placement and control influx and migration of fluid into the cement column. These additives usually are organic and polymeric materials, lignite and cellulose derivatives, which can cause severe formation damage by fines' migration (Refs. 9 through 13). Well construction in North of Monagas area is done using oil-based fluids, highly densified to provide physicochemical stability to critical rock formations (deep formations, swelling clays and high pressure and temperature). Spacers to remove oil based mud of 18 ppg and densified cement slurries are commonly used (Ref. 14). This kind of materials can contribute significantly to severe formation damage by fines' migration, wettability alterations, insoluble solids precipitation and emulsion formation among others.
A comprehensive analysis scheme devised in order to cope with solid precipitation during oil production is being currently followed in many Venezuelan production units. Three goals from the scheme are: 1. Identification of remedial actions based on deposits characterization, 2. Selection of the most efficient dispersant additive, 3. Choice of the proper solvent mixtures for cleaning production strings, when organic types were assessed to be the main components of the deposit. This paper addresses these aspects with examples taken from a field located in eastern Venezuela. Introduction Solid precipitation has been experienced in many Venezuelan production units since the past decade1–3. A preventive and corrective global approach to handle the problem has been proposed1,4,5, based on the historical production records and physicochemical characterization of fluids and available solid deposits. For operationally unstable crude oils (production records showed solids precipitation), one of the key issues is the identification of chemical dispersants able to prevent solids adhesion to pipe walls. One way to find the proper additive(s) is to titrate a sample of the dead crude with an alkane precipitant, including a certain dosage of a commercial inhibitor. This titration is commonly carried out at ambient conditions, following published methodologies6,7. However, recent findings suggest that this procedure when done at high temperature improve the detection limits and the reproducibility of this determination8. When field workovers made available solid deposits, expanded possibilities are then open through their analysis in order to get to understand the origin of the problem and, to improve the remedial and preventive actions to be taken for each particular case. The deposit analysis scheme has been described previously3,9,10 and a picture of the general causes for precipitation can be achieved by correlating deposits compositional information got from wells drilled into the same reservoir. Another benefit derived from deposit characterization is the identification of the proper solvent mixtures to effect their redissolution, when organic types were the main components of the solid. Ultrasonic, accelerated extraction and simple redissolution experiments were carried out to this end. Simple redissolution experiments proved to be the best trade-off. Some results in this regard have been published10, and more details are to be found within the discussion from this paper. Examples from a field study located in eastern Venezuela illustrate the application and benefits derived from the titration, characterization and dissolution protocols, which will be also covered by themselves within this paper. Crudes and deposits were sampled during the past 2 years from the field identified during the ensuing discussion like "Field PST".
This paper was prepared for presentation at the 1999 SPE Latin American and Caribbean Petroleum Engineering Conference held in Caracas, Venezuela, 21–23April 1999.
In Venezuela, at Maracaibo Lake (west) and Anzoategui-Monagas (east), there are reservoirs that have been under production for more than 30 years, which are presenting now low reservoir pressure. This situation has prompted the search for new technologies to maintain or increase the production in these zones. So far the technology that has been more successful is horizontal drilling, including the use of aerated drilling fluids and multilateral wells. Up to date there are some 700 horizontal wells in Venezuela, with an average initial production of 800–1000 BPD per well. The majority of these wells are drilled setting a casing on top of the productive zone and changing the drilling fluid to one designed to properly seal and minimized formation damage. There are areas in Lake Maracaibo where the equivalent reservoir pressure is less than the density of water; the horizontal section is drilled with a mixture of air and water based fluid to achieve 3.5 – 6.5 ppg at bottom. Multilateral horizontal technique has started with the design of wells with two arms, each in a different sand and opposite direction. Production can be commingled or set separately if needed. This paper provides details of field applications of these technologies, production data and future projections. Introduction Horizontal drilling, as an emerging technology, had made possible an improvement in production from depleted reservoirs or from remnant narrow sands. In Venezuela, horizontal wells are drilled in two main areas: Maracaibo Lake in the west region and Anzoategui-Monagas in the east region, see Fig. 1. Since the beginning of horizontal drilling in Venezuela, more than 600 wells had been drilled and over 1500 wells are expected for the year 2000 as Fig. 2 shows. These horizontal wells include mostly reentry wells to increase production of depleted reservoir from new exposed sands and new wells in narrow sands in order to incorporate remnant crude from them. From now on in this paper whenever horizontal wells are mentioned it should be taken as including both, new and reentry wells. Due to the increase in horizontal drilling, technical aspects of the application of this technology in Venezuela are presented in this paper focused mainly in Maracaibo Lake. Improvement of application of horizontal drilling with other technologies, as aerated drilling, will be included. Horizontal Wells Miocene sands are poorly consolidated and start at an average depth of 2500 to 3500 feet, with an average reservoir pressure of 1100 psi. The firsts horizontal wells drilled in these sands were done setting a casing above the productive zone and continued drilling with an excessive density of barite densified fluid until the end of the horizontal section. The production of these wells drilled up to 1994 was 1,5 times a vertical well. Laboratory Tests. Cuttings of these wells were examined using an electron diffraction microscope which showed that all the pore spaces, within certain depth, were invaded by barite. Formation damage from an inadequate formulation of drilling fluid, specially in the choice of kind and particle size of the bridging agent, was causing the low productivity of these wells. Horizontal wells have substantially longer damaging contact time with the drilling fluid. Furthermore, draw down of the productive zone may not be enough to remove the damage. While the external filter cake is formed during the drilling process, a spurt loss occurs that carries fluid and solid particles into the face of the formation, thus forming an internal filter cake. Once the filter cake is formed, external and internal, only filtrate will penetrate in the formation due to hydrostatic pressure.
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