Sand Control In Horizontal Wells

Sand control in horizontal wells

The evolution of sand control from historical water wells to recommendations for 15,000 ft+ horizontal oil/gas wells.

This summary article backgrounds the need for formation sand entry prevention in downhole producing wells from man's first dumping of rocks into water wells drilled with rock or iron tools. Thousands of years later, the oil/gas industry invented basic gravel pack and sand screen methods to prevent inflow of unconsolidated formation sands. Now the drilling of horizontal boreholes over 15,000-ft long has created ever new challenges and need for sand control in extremely long sand intervals.

The discussion covers this story with: 1) A historical review; 2) Basic barefoot, gravel pack and stand-alone screen completions; 3) Selecting sand control type for a horizontal well; and 4) Innovation for future use.

HISTORY OF SAND CONTROL

Way, way back, when early humans needed water, they dug water wells with their hands. Then they needed more water, so they began using tools to drill into sand beds. They found that they could work faster and go deeper by using a heavy rock or iron as a percussion tool, like a yo-yo, to cut through to the water bearing sand.

Water-well technology. Sand problems were born. Loose sand was coming up with the water or falling to the bottom of the well, stopping the water flow. They were able to prevent the problem by filling a small part of the well with large rocks.

Much beyond the rock-dumping era, better ways to stop sand production and maintain high water production rates were developed. By the late 19th and early 20th centuries, they cut holes in liners to stop sand production, and later found they could drop gravel down the annulus to form gravel packs that gave even better results. Oil companies quickly modified sand control techniques to handle deeper and dirtier wells.

Oil industry advances. Perforated liners were popular in the early 20th century, without much concern about the sizes of perforations to be used. Then academia jumped on the oil wagon with such people as C.J. Coberly, who published his work on "Selection of screen openings for unconsolidated sands" in 1937. 1 This was a starting point for technical means of designing sand control tools.

Great improvements were developed by the oil/gas industry despite politics and prejudices. Some of the major sand control techniques developed during the second half of the 20th century were: sand consolidation, slotted liners, wire wrapped screens, prepacked screens, premium screens, gravel packs, frac packs, high-angle-hole gravel packing and horizontal-well gravel packing.

The most challenging of these has been controlling sand in horizontal wells, as drilling technology is now able to drill to distances of more than 15,000 ft from heel to toe. For the uninitiated reader, "heel" is the beginning of the horizontal wellbore and "toe" is the end of the horizontal wellbore.

CONTROLLING SAND IN HORIZONTAL WELLBORES

Regarding barefoot completions, the simplest and least expensive completion is to put a well on production without anything left in the productive interval. Without casing, liner, screen or tools therein, we rely on the sandstone being strong enough, and shale beds stable enough, to resist changing pressures as the well is depleted. This works very well in strong sandstones, dolomites and limestones, but will fail in weaker sandstone, or where there is tectonic activity.

Sometimes it is impossible to understand why sand begins to be produced from a very strong sandstone, but the most common cause is the onset of water production. There may be a chemical reaction between extraneous water and natural cementing agents of the oil bearing formation. This should always be tested and anticipated by core tests.

One barefoot completion in a horizontal well off Western Australia began producing sand after a year or so of production, and it was found that the heel had collapsed. When the operator attempted a clean-out it couldn't get into the hole, and no explanation could be found. In another case, a vertical well near the Gulf of Mexico stopped producing, and a downhole camera found that the casing was sheared at one point. The conclusion was that some tectonic activity caused two layers to slide across each other.

Stand-alone-screen completions. A Stand-Alone-Screen (SAS) completion simply employs some type of screen or liner positioned inside a productive interval. Prepacked screens are expensive, so most stand-alone-screen completions are done with slotted liner, wire-wrapped screen or premium screen. One of the first SAS horizontal-well completions was in the Helder field offshore the Netherlands. It was done with a large-diameter prepacked screen, resulting in negligible skin factor and successful sand control.

Any screen in an open hole will scrape along the low side of the drilled hole and pick up drilling fluid solids, cuttings, fluid loss agents and formation sand that can reduce screen permeability. Such screen damage may have only a minor effect on well productivity, but may force fluid to flow through short screen sections, eroding holes through the screen, commonly called "hot spots." Centralizers or shrouds help prevent this but add cost and bulk.

A stand-alone-screen is likely to have non-uniform inflow, as it is easier for fluid to flow into the screen that is nearest to the heel of the wellbore. This should not be a concern in most wells, but it may cause screen erosion and premature failure. Such problems are most likely to occur in wells where watercut is high, and fluid flow friction from the toe of a well restricts production more from the far end of the borehole than from near the heel. This problem can be reduced by using flow restriction systems, such as the ResFlow system, trade marked by Reslink, that evens inflow velocity throughout the length of the horizontal wellbore. 2

Perforations in a cased hole should only be toward the high side of the casing, because low-side perforation tunnels are likely to be loaded with mud solids, fluid loss solids, and sand/ dirt, as affected by gravity. High-side perforations will be somewhat self-cleaning and are less likely to collapse.

The velocity of producing fluid entering radially into a screen may be so low that sand will remain in the annulus, and sand dunes may completely fill some sections of the annulus. This adds noticeable skin and may stop all production from the wellbore