partial penetration, Oil & Gas Glossary

What Is Partial Penetration?

Partial penetration (also called limited entry or restricted entry) is a well completion configuration in which the perforated or open interval of the wellbore penetrates only a portion of the total pay thickness, rather than the complete reservoir interval. This creates a zone of converging flow near the wellbore where fluid must travel vertically as well as radially to reach the limited entry point, resulting in an additional pressure drop called the partial penetration skin. The partial penetration skin factor is always positive and reduces well deliverability compared to a fully penetrating completion.

Key Takeaways

Partial penetration occurs when perforations or an open-hole interval cover less than 100 percent of the net pay thickness, typically expressed as the penetration ratio hw/h.
The incomplete entry forces spherical and hemispherical flow convergence near the wellbore, generating a positive skin (partial penetration skin, Spp) that reduces productivity index.
Engineers intentionally design partial penetration to prevent gas coning from a gas cap above or water coning from an aquifer below the oil zone.
Pressure transient tests in partially penetrating wells show a characteristic negative half-slope (spherical flow period) on a log-log plot before radial flow develops.
Vertical permeability (kv) has a dominant influence on partial penetration skin: low kv dramatically increases Spp and can make a well uneconomic even at moderate penetration ratios.

How Partial Penetration Works

In a fully penetrating well, reservoir fluid flows radially from all points in the pay thickness toward the wellbore, distributing the pressure drawdown over the entire vertical interval. When only a fraction of the pay is open to flow, fluid from the unperforated portions must converge vertically toward the open interval before turning radial. This three-dimensional converging flow pattern generates a higher velocity near the completion interval, which means a higher pressure drop and therefore an additional skin. The Brons and Marting correlation, published in 1961, remains the most widely used analytical method for estimating partial penetration skin from the penetration ratio (hw/h), the reservoir thickness (h), the wellbore radius (rw), and the permeability anisotropy ratio (kv/kh).

The magnitude of partial penetration skin increases sharply as the penetration ratio decreases and as vertical-to-horizontal permeability anisotropy (kv/kh) drops. In a formation with kv/kh of 0.1 and a penetration ratio of 0.3, the partial penetration skin can exceed 20, equivalent to severe damage skin. Engineers must therefore weigh the coning risk (which favours limiting entry) against the productivity penalty before choosing a completion interval. In tight or laminated reservoirs where kv/kh is very low, even moderate partial penetration can impose prohibitive skin values.

The distinction between partial penetration and selective perforation in a layered reservoir is important. Selective perforation means choosing specific layers to perforate for zone isolation, inflow control, or conformance purposes; the unperforated intervals may be intentionally bypassed rather than being part of the pay connected to the perforated zone. Partial penetration, in the strict pressure-transient sense, refers to a single continuous pay interval where only part of the thickness is open to flow. Both create convergent flow and positive skin, but their geological interpretations differ.

Fast Facts: Partial Penetration

Penetration ratio (hw/h): Open interval thickness divided by total pay thickness; ranges from near 0 to 1.0
Skin type: Partial penetration skin (Spp); always positive, adds to total skin in well test analysis
Key correlation: Brons and Marting (1961); Papatzacos (1987) for eccentric placement
Flow regime signature: Spherical/hemispherical flow on log-log pressure derivative plot (slope = -1/2)
Critical parameter: kv/kh ratio; Spp is inversely proportional to square root of kv/kh
Main application: Coning control in oil reservoirs with active gas caps or bottom water
Typical penetration ratios: 0.2 to 0.5 for coning avoidance; 0.5 to 0.8 for wellbore stability
Productivity impact: PI reduction of 20 to 60 percent common at penetration ratios below 0.4 with low kv/kh

Field Tip:

When interpreting a pressure buildup test from a suspected partially penetrating well, look for the spherical flow period as a straight line with slope -1/2 on the log-log derivative plot, appearing before radial flow stabilises. If you proceed straight to radial flow analysis and ignore the spherical period, you will underestimate skin and potentially attribute the excess pressure drop to damage rather than geometry. Correctly identifying the flow periods allows you to back-calculate kv/kh, which is otherwise difficult to measure without a vertical interference test.

Reasons for Intentional Partial Penetration

Water or gas coning is the most common reason engineers deliberately limit completion intervals. In a reservoir with an oil column underlain by a strong aquifer, perforating the bottom of the oil zone accelerates water encroachment toward the wellbore, quickly destroying oil productivity and raising water handling costs. By perforating only the upper portion of the oil column, engineers reduce the vertical pressure gradient that drives water upward. The optimal placement of a limited entry interval for coning control is typically the upper 20 to 40 percent of the oil column, as far as practical from the water-oil contact. Similar reasoning applies to oil reservoirs with gas caps, where perforating near the gas-oil contact would induce gas coning.

Wellbore stability and mechanical integrity provide secondary reasons for partial penetration. In unconsolidated sands or naturally fractured formations, opening the full pay interval may cause sand production or uncontrolled fracture connectivity. Limiting entry to a stable competent interval reduces these risks. In horizontal wells, partial penetration in the vertical sense is less relevant, but the analogous concept of limited-entry perforating in horizontal completions is used to create distributed inflow and delay water or gas breakthrough from heel-to-toe heterogeneities.

limited entry: common field term for any completion where only part of the pay is open; used interchangeably with partial penetration in pressure transient literature
restricted entry: emphasises the deliberate restriction of inflow to control coning or improve conformance
partial completion: broader term encompassing mechanical reasons for opening only a portion of the reservoir
selective perforation: perforation of specific layers in a multilayer system, related but distinct from single-interval partial penetration

Frequently Asked Questions About Partial Penetration

How is partial penetration skin calculated in practice?

The most common approach uses the Brons and Marting correlation, which requires the penetration ratio (hw/h), the dimensionless wellbore radius (rw/h), and the permeability anisotropy ratio (kv/kh). The calculation is available as a chart solution or embedded in well test analysis software. For eccentric placement of the open interval (not centred in the pay), the Papatzacos (1987) solution provides more accurate results. Modern pressure transient analysis software fits a complete flow model that includes the spherical flow period, allowing simultaneous determination of horizontal permeability, vertical permeability, and skin from a single buildup test.

Does partial penetration skin change over time?

The geometric component of partial penetration skin is fixed by the completion geometry and does not change with time. However, the effective skin measured in a buildup test may appear to decrease over time as the spherical flow radius expands and the test begins to sample more of the vertical permeability structure. Damage skin can also change with production history. Engineers must therefore specify the test duration carefully so that radial flow is achieved and the full skin can be separated from the spherical flow transient.

Can partial penetration be used in horizontal wells?

Yes, though the geometry differs. In horizontal wells, limited-entry perforating spaces perforation clusters along the lateral at intervals to create multiple discrete inflow points, rather than one continuous open interval. This delays breakthrough of water or gas at any single location and improves sweep along the wellbore. The pressure drop at each cluster resembles spherical flow convergence, and inflow control devices (ICDs) are often used alongside limited-entry perforating to further equalise inflow along the lateral.

Why Partial Penetration Matters in Oil and Gas

Partial penetration is a fundamental concept linking completion design to reservoir engineering and pressure transient analysis. Getting the penetration ratio right can mean the difference between an oil well that produces above solution gas ratio for years and one that rapidly cones water or gas and becomes uneconomic. Equally important is correctly identifying and quantifying partial penetration skin during well testing: misinterpreting the spherical flow period as damage leads to unnecessary stimulation jobs that do not improve productivity. As formations become more challenging, with thin oil columns, active aquifers, or low vertical permeability, the decision of how much of the pay to perforate carries increasing financial consequence, making partial penetration one of the key completion design variables in any reservoir development plan.

Partial Penetration: Well Completion and Pressure Transient Analysis