Live oil

Live oil with dissolved methane does not follow the above correlations as methane relaxes by a spin-rotation mechanism, even when dissolved in liquid hydrocarbons [13]. The Ti relaxation time as a function of rj/T is illustrated in Figure 3.6.2 for different gas/oil ratios expressed in units of m3 m-3 as a parameter. The solid line is the fit for zero gas/oil ratio and is given by Eq. (1). 

The assessment of surfactant structures and optimal mixtures for potential use in tertiary flooding strategies in North Sea fields has been examined from fundamental investigations using pure oils. The present study furthermore addresses the physico-chemical problems associated with reservoir oils and how the phase performance of these systems may be correlated with model oils, including the use of toluene and cyclohexane in stock tank oils to produce synthetic live reservoir crudes. Any dependence of surfactant molecular structure on the observed phase properties of proposed oils of equivalent alkane carbon number (EACN) would render simulated live oils as unrepresentative. 

Recombined oil (also referred to as live oil ) was prepared by saturating the oil with methane gas in recombination equipment connected to the inlet end of the core holder. Produced fluids were collected into a graduated cylinder placed on an electronic balance for measuring the oil production rate. An automated data acquisition system was employed for recording of the oil-production rate and the differential pressures in each segment during the flow experiment... 

Preparation of Recombined ( Live ) Oil. Prior to the start of flow experiments, each oil sample was cleaned of its suspended materials. The oil was then recombined with methane gas in the recombination equipment at a pressure of 4.83 MPa. During this recombination process, the solution gas—oil ratio (SGOR) was monitored periodically, and when a constant SGOR was established, the oil was considered to be fully saturated. This usually took about 5—6 days of continuous mixing. Properties of recombined oils are reported in Table II. 

Porous Medium. Ottawa sand was used as the porous medium for flow experiments. The porosity and absolute permeability to the water were determined prior to the start of flow experiments (Table I), and the core was then resaturated with live oil by displacing the water. During the resaturation process, almost 98% of the water was displaced. This value of irreducible water saturation (2%), although much lower than the field values, is not exceptional in laboratory tests of this nature. 

Procedure. The sequence of flow experiments for each oil started with the core saturated with the live oil at saturation pressure, 4.83 MPa. The pressure at the inlet end was always maintained at 4.83 MPa. A back-pressure regulator (BPR) was used to maintain constant pressure at the outlet end, and this pressure was reduced in steps of approximately 0.34 MPa, starting from the saturation pressure. The term pressure drawdown will be used in the following discussion to denote the pressure difference between the inlet end and the outlet end of the sand pack. The pressure drawdown reaches its maximum value when the pressure at the outlet end becomes equal to the atmospheric pressure. 

In order to study asphaltene precipitation from live oil systems at reservoir conditions and to study effect of pressure on amount of asphaltene precipitation, experimental set up was assembled. Figure 11 is a schematic diagram of the setup used. The major components of the set up include 1) JEFRI positive displacement pump, 2) recombination cell, 3) solvent transfer (pistoned) cell, 4) stainless steel (60 micron) In-line filter, 5) air bath equipped with temperature controller and rocking mechanism. Positive displacement pump was used either to inject gas during recombination of oil or to inject CO2 at constant pressure into recombination cell to prepare desired CO - recombined oil mixture. 

Initially, the known amount of tank oil was taken into the recombination cell. The dead oil was recombined at 1705 psia and 80°F. The composition of live oil is given In Table 3. The solvent (CO ) was then Injected from the solvent transfer cell into the recombination cell (which also acted as asphaltene cell) at constant pressure. The amount of CO injected was determined from the pump reading. The resulting CO2-0II mixture was rocked for several hours to achieve good mixing. The mixture was then passed through a high pressure stainless steel... 

Materials. Several fluid properties, listed in Table 1, were determined at a temperature of 23 °C and a pressure of 1000 psig (dead oil indicates no gas saturation while live oil indicates nitrogen gas saturated). A compromise was chosen between extreme Canadian reservoir conditions and the limiting operating conditions of the laboratory equipment. All experiments were conducted at a pressure of 6900 kPa (1000 psig) and at room temperature (23 °C). Berea rock was used as core material. 

Gonzalez et alP modeled the asphaltenes precipitation in live oils with the (Perturbed Chain-Statistical Associating Fluid Theory (PC-SAFT) see Chapter 8 for additional material. It is not an easy task to apply a complicated model such as PC-SAFT to systems consisting of a very large number of chemical... 

The viscosity of the live oil, water and polymer solution was 950 mPa-s, 1 mPa-s and 25 mPa-s, respectively. The relative permeability curves used to achieve the history match for the water-flood and subsequent polymer floods, are shown in Figure 3. Based on the shape of the relative permeability curves, the core is water wet, = 0.1. The water and polymer coreflood parameters were used to calibrate the field scale simulation model described in the next section. 

The experiments were conducted at a constant flow rate of 1 PV/d, temperature of 97°F, 290 psi core pressure, and 1,000 psi net overburden. The materials used included live oil (764 cP), a 12 Darcies synthetic reservoir core, and a synthetic water representative of locally available well water. In the initial waterflood, reservoir brine was injected. 

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