MARCO TEORICO

CHOPS is widely used as production approach in unconsolidated sandstones.

CHOPS involves “the deliberate initiation of sand influx during the completion procedure, maintenance of sand influx during the productive life of the well, and implementation of methods to separate the sand from the oil for disposal. No sand exclusion devices (screens, liners, gravel packs, etc.) are used. The sand is produced along with oil, water, and gas and separated from the oil before upgrading to a synthetic crude” (Dusseault, 2007).

The first discoveries in the Canadian heavy-oil belt were made in the Lloydminster area in the late 1920s. High asphaltene-content heavy crude, an ideal feedstock for asphalt products, has been produced since that time. Pump jacks were limited by slow rod-fall velocity in the viscous oil to a maximum of 8 to 10 m³/day of production, usually less. Small local operators found that wells which continued to produce sand tended to be better producers, and efforts to exclude sand with screens usually led to total loss of production.

The sharp oil price increases in the 1970s and 1980s led to great interest in heavy-oil-belt resources. Many international companies arrived and introduced the latest screen and gravel-pack technology. However, in most of cases, the productivity was reduced remarkably or total stopped. The development of progressing cavity pumps (PCPs) since the 1980s changed the non-thermal heavy oil industry in Canada. The production rate limits of beam pumps were no longer a barrier. Sand became an asset because more sand meant more oil.

More highly integrated sand separation, transportation, and disposal methods were developed (Dusseault, 2002). To date, deliberate massive sand influx has been used only in unconsolidated sandstones (Cv ≈ 30%) containing viscous oil (μ > 500 cP). It has been used almost exclusively in the Canadian heavy-oil belt and in shallow (< 800 m), low-production-rate wells (up to 100 to 125 m³/day).Also, from 12~20% original oil in place (OOIP) can be developed.

However, wider acceptance of this technology is still expected. The reasons for

mechanisms, difficulty in production predictions, complexity in implementation, and the need for high level of sand management and disposal strategies.

CHOPS technology is used as recovery heavy oil’s reservoirs to increase porosity and permeability of the formation significantly and improve flowability of heavy oil by inducing sand production to form wormhole net. Where Vertical wells are drilled into the zone of interest, and sand production is encouraged using special screens and slotted liners shown in Figure 2-12 and Figure 2-13.

As production continues, substantial quantities of sand, 1% to 8% of the total volume, are produced along with the oil. High-porosity, high-permeability channels, known as wormholes, penetrate into the formation and become preferential production paths (Drebit and Tesciuba, 2008). These wormholes tend to develop and grow in the weakest sand and propagate toward the highest pressure gradient. The recovery mechanism of CHOPS includes:

wormhole net formed due to a great deal of sand production, steady foamy oil flow, elastic expansion of reservoir, compressive derive of overlying formation, and edge/bottom water providing drive energy, etc (Wang et al., 2005).

Figure 2-12: Forming wormholes using special screens

Figure 2-13: Producing heavy oil with sand

CHOPS has special requirements for reservoirs, such as shallow buried depth, unconsolidated formation, easy-to-produce sand, low reservoir pressure, moderate solution gas-oil ratio, and high oil viscosity with some edge-bottom water providing drive energy (Wang et al., 2005). However, regardless of the reservoirs condition, CHOPS will be inevitably faced with many problems such as sand production and surface oil/ sand treatment. This will simultaneously increase oil production cost. Consequent engineering technical problems mainly include:

 malfunctioning/ damaging of production tools, for example, progressing cavity pump is clogged or stuck, which may reduce its service life;

 wellbore sand settling will result in work-over and frequent sand washing operation, which will increase work-over expense greatly;

 surface gathering pipelines are easy plugged during heavy oil production with sand; and

 oil/ sand separating and processing equipments are required on surface due to sand production with produced crude oil.

There are several mechanisms responsible for the production rate enhancement in CHOPS wells:

 When the sand matrix is unconstrained (no screens or other impediments) and is allowed to move with the viscous fluids into the well bore, the basic permeability is increased, and the oil mobility is thereby enhanced.

 Continued sand production from a CHOP well leads to the growth of a disturbed zone, most likely a channelled and remoulded zone filled with slurry of sand, water, oil and gas. The zone increases porosity and permeability, and the well behaves as if it has an increased drainage radius. Production enhancement from this effect alone should approach a factor of 4 or 5 after large quantities of sand have been produced. Late in life this could be 1,000 to 15,000 bbls of sand per well.

 CHOPS production uses foamy oil drive from the solution gas with intentional sand production. Wells are subjected to aggressive drawdown, and gas evolves as bubbles in the porous matrix. However, a continuous gas phase is not formed due to the high fluid viscosity; gas remains as bubbles that expand in response to pressure decline during flow. Hence, the bubbles act as an “internal drive” driving the slurry to the well at a greater velocity than predicted by conventional fluid flow theories. Foamy oil develops in a zone that propagates outward from the well, following the growth of the disturbed and remoulded zone. This extends the zone of highest pressure gradient far from the well, where it destabilizes more sand. Operating below the bubble point means a dramatic increase in production rate and recovery.

 Heavy oil reservoirs can have high skin effects due to plugging of pore throats with precipitated asphaltenes and mobilised fine-grained particles and clays. CHOPS continually shears and disturbs the sand grains, which prevents pore throat plugging. As the disturbed zone of hyper-porosity and permeability extends away from the wellbore, the wellbore skin becomes increasingly negative.

Figure 2-14 shows the history of Luseland Field in Saskatchewan that was converted to CHOPS production in 1994~1998, showing the huge increase of production rate after applying the CHOPS technology.

Figure 2-14: Field improvement through CHOPS (Dusseault, 2002)