Page 52 - Petroleum and Gas Field Processing

with and dissolving the carbonate. This creates irregular highly conductive
channels, known as a wormholes, for easier flow of fluid from the
formation to the well.
For sandstone matrix acidizing, a solution of hydrofluoric and
hydrochloric acids (known as mud acid ) is injected into the formation to
dissolve clay and, to some extent, silica. This removes the damage to
restore the near-wellbore permeability.
Fracturing is performed mostly on wells completed in very tight (very
low permeability) formations. In carbonate formations, a fracturing fluid
followed by HCl is injected at pressures exceeding the formation fracturing
pressure. The fracturing fluid initiates a fracture and the acid reacts with
the carbonate walls of the fracture, leaving the walls as rough surface.
Therefore, when the pressure is reduced, the fracture will not close and will
provide a very conductive passage for fluids to flow from the formation
into the well. This operation is known as acid fracturing. Fracturing tight
sandstone formations is known as hydraulic fracturing. As with carbonate
fracturing, a fracturing fluid is injected at high pressure to initiate the
fracture. The fracture is kept open by filling it with highly permeable,
high-compressive-strength sand known as proppant.

Sand Control
When wells are completed in unconsolidated or weakly consolidated
formations, sand is likely to be produced with the fluids. Sand production
is a very serious problem. Produced sand erodes subsurface and surface
equipment, necessitating very costly frequent replacement. Sand also
settles in the bottom of the well and in surface processing facilities. This
requires periodic shutdowns to clean the well and facilities.
Methods are available to control or prevent sand production. These
are classified as mechanical retention methods and chemical,or plastic,
consolidation methods. The simplest method of mechanical retention
involves the installation of screens opposite the producing zone. Screens,
however, are subject to erosion and corrosion and would need to be
replaced frequently. Therefore, screens are used only as a temporary
solution to sand production problems. The best and most commonly used
method of mechanical sand control is known as gravel packing. In simple
terms, the method involves the installation of a screen having a smaller
diameter than the casing diameter opposite the producing zone. The
annular space between the casing and screen is packed with specially sized
gravel (sand). Formation sand will bridge against the gravel, which bridges
against the screen. This prevents formation sand from flowing with the
fluids into the well.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.