📄 earth_science

Investigation of interlayer interference regulation in offshore high-water-cut reservoirs with horizontal-vertical combined well patterns

This study demonstrates that combining well pattern adjustments with separate-layer injection–production is more effective than simple production shutdowns for regulating interlayer interference and improving recovery in offshore ultra-high water-cut heavy oil reservoirs by mobilizing medium- and low-permeability layers and reducing preferential flow in high-permeability zones.

Original authors: Kun Qian, Longzhi Wu, Yanfeng He, Xiangji Dou, Xingbang Meng Meng, Shenglai Yang

Published 2026-08-11

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Kun Qian, Longzhi Wu, Yanfeng He, Xiangji Dou, Xingbang Meng Meng, Shenglai Yang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth's crust as a giant, multi-layered sponge cake, but instead of vanilla and chocolate, the layers are made of rock with different textures. Some layers are like loose sand, letting water and oil flow through them easily; others are like dense clay, making it a tough slog for fluids to get through. For decades, oil companies have been trying to squeeze every last drop of oil out of these "reservoirs" by injecting water to push the oil toward wells. But here's the tricky part: just like water rushing down a steep slide, the injected water loves the easy path. It zooms through the loose, sandy layers, leaving the oil trapped in the dense, clay-like layers behind. This creates a messy situation called "interlayer interference," where the fast layers hog all the action, and the slow layers get ignored. By the time a reservoir is "ultra-high water cut" (meaning almost all the liquid coming out of the well is water, not oil), the easy layers are exhausted, and the oil is stuck in the hard-to-reach spots between the wells. The big question for engineers is: how do you fix a system that's already broken without just making the problem worse?

This paper dives into that exact puzzle, focusing on offshore oilfields where the rock is particularly messy and the oil is thick and sticky. The researchers, led by Kun Qian and Longzhi Wu, didn't just look at how to start a new oil field; they looked at how to fix an old one that was already in trouble. They set up a giant, 3D physical model of a real offshore reservoir in the Bohai Sea, complete with three distinct layers of rock, and ran it like a high-tech video game. They tested a series of "adjustment schemes," essentially trying different ways to rearrange the wells and the flow of water to see if they could wake up the sleeping oil in the tough layers. Think of it as trying to get a stubborn group of friends to share a pizza equally when one friend keeps grabbing the biggest slices. They tested adding new wells, shutting down old ones, and changing who was eating and who was serving, all while watching how the "interference" between the layers changed.

The story of their findings is a tale of trial, error, and a surprising "aha!" moment. First, they tried Scheme 1, which was like adding a super-fast straw (a horizontal well) directly into the layer that was already eating the most pizza (the high-permeability layer). The result? That layer got even more oil, but the other layers got even less. The "interlayer interference" actually got worse because the fast layer just sucked up even more water, leaving the slow layers completely dry. It was a classic case of the rich getting richer while the poor got poorer.

Next, they tried Scheme 2, which was a bit more gentle. They added that same super-fast straw but then decided to plug the main tap (shut in the central production well) in the fast layer to force the water to look for other paths. This helped a little bit. The slow layers got a tiny bit more oil, and the interference dropped slightly, but it wasn't a magic fix. The fast layer was still the boss, and the water just found a way to sneak back in. It was like telling the greedy friend to slow down, but they still managed to grab the biggest slice.

Then came the real game-changer: Scheme 3. This was the "staggered well pattern" combined with "separate-layer injection." Imagine rearranging the seating chart so that the friends who were sitting in a straight line (a row well pattern) are now sitting in a zig-zag or triangle (staggered). But the real magic was that they gave each layer its own dedicated waiter (separate-layer injection). Instead of one big water hose hitting all layers at once, they controlled the water flow for each layer individually. The results were spectacular. In their physical experiments, the total oil recovered jumped by nearly 20 percentage points compared to the previous attempt. In their computer simulations, it was a similar massive leap. The "interlayer interference coefficient" (a number measuring how much the layers were fighting each other) plummeted. The water finally started visiting the slow, stubborn layers, sweeping up the oil that had been left behind. The oil distribution became much more even, like a perfectly shared pizza.

They also tested Scheme 4, which involved swapping the roles of some wells (turning injectors into producers and vice versa) without changing the overall layout. This helped a bit more than the simple shut-in, but it still couldn't beat the "staggered plus separate-layer" approach. It was like rearranging the furniture in a room; it helped the traffic flow a little, but it didn't fix the fact that the room was still too crowded in one spot.

The authors conclude that you can't just tweak one thing to fix a broken system. Adding a well or shutting one down alone isn't enough to solve the mess of a highly uneven reservoir. The winning strategy, they found, is a two-pronged attack: change the geometry of the wells to a staggered pattern and take control of the water injection for each layer separately. This approach doesn't just squeeze a little more oil out; it fundamentally changes how the reservoir behaves, allowing the difficult layers to catch up and ensuring that the remaining oil is mobilized effectively. It's a reminder that sometimes, to get the best result, you have to stop treating the whole system as one big block and start giving each part the specific attention it needs.

Technical Summary: Investigation of Interlayer Interference Regulation in Offshore High-Water-Cut Reservoirs with Horizontal-Vertical Combined Well Patterns

Problem Statement
Offshore heavy oil reservoirs in the Bohai Sea have entered an ultra-high water cut stage characterized by severe development imbalance. Due to strong vertical heterogeneity, high crude oil viscosity, and long-term water flooding, injected water preferentially migrates through high-permeability layers, forming dominant flow channels. This phenomenon leads to insufficient utilization of medium- and low-permeability layers, significant remaining oil accumulation between wells, and intensified interlayer and interwell interference. While previous studies have addressed individual adjustment measures (e.g., horizontal wells, well pattern changes), there is limited understanding of the progressive optimization process based on the development response of preceding stages. Specifically, the evolution of reservoir performance during sequential well pattern adjustments and the specific effects of different strategies on interlayer interference and remaining oil redistribution require further investigation.

Methodology
This study employed a combined approach of three-dimensional physical simulation experiments and numerical simulations using consistent experimental parameters to investigate progressive well pattern adjustments in a typical offshore heavy oil reservoir.

Experimental Model: A physical model was constructed with three vertically stacked layers representing the target reservoir's heterogeneity: an upper low-permeability layer (1000 mD), a middle high-permeability layer (5000 mD), and a lower medium-permeability layer (3000 mD). The system operated at 65°C with a constant injection rate of 0.5 mL/min.
Evaluation Metrics: Development performance was assessed using the recovery factor, the interlayer interference coefficient (defined as $\alpha = (Q_{ref} - Q_i) / Q_{ref}$ , where $Q_{ref}$ is the production of a reference separate-layer scheme and $Q_i$ is the production of the evaluated scheme), and remaining oil distribution.
Experimental Schemes:
- Reference Scheme: A row well pattern with separate-layer injection-production (serving as a benchmark).
- Scheme 1: Introduction of a horizontal production well into the high-permeability layer at 80% water cut.
- Scheme 2: Shutting in the central production well of the high-permeability layer at 90% water cut (following Scheme 1).
- Scheme 3: Implementation of a staggered well pattern combined with separate-layer injection-production at 90% water cut (following Scheme 2).
- Scheme 4: Conversion of injection-production roles (changing specific producers to injectors and vice versa) at 90% water cut (following Scheme 2).

Key Results

Scheme 1 (Horizontal Well Infill): Adding a horizontal well to the high-permeability layer increased the overall recovery factor, but the improvement was almost entirely contributed by the high-permeability layer. The medium- and low-permeability layers saw limited improvement, and the interlayer interference coefficient increased, indicating intensified preferential flow and development disparity.
Scheme 2 (Well Shut-in): Shutting in the high-permeability production well slightly increased the overall recovery factor and reduced the rate of increase in the interlayer interference coefficient. However, because commingled injection-production was maintained, preferential flow persisted, and the fundamental development imbalance was not resolved.
Scheme 3 (Staggered Pattern + Separate-Layer Injection): This scheme yielded the most significant improvements. In physical experiments, the recovery factor increased by 19.37 percentage points compared to Scheme 2, and the interlayer interference coefficient dropped from 0.38 to 0.11. Numerical simulations showed a similar trend (19.27 percentage point increase in recovery; coefficient drop from 0.58 to 0.14). This approach successfully mobilized medium- and low-permeability layers, resulting in a more uniform remaining oil distribution and coordinated development across all layers.
Scheme 4 (Injection-Production Conversion): Adjusting injection-production relationships improved local flow and increased recovery from medium- and low-permeability layers compared to Scheme 2. However, without separate-layer control or a fundamental change in well pattern geometry, preferential flow persisted, and the overall development performance remained inferior to Scheme 3.

Significance and Claims
The paper claims that individual adjustment measures are insufficient to resolve development imbalances in the ultra-high water cut stage of strongly heterogeneous reservoirs. Instead, a progressive adjustment strategy based on the development response of previous stages is necessary. The study concludes that the combination of a staggered well pattern and separate-layer injection-production is the most effective strategy identified. This approach simultaneously optimizes well pattern configuration and development modes, effectively regulating interlayer fluid allocation, weakening preferential flow in high-permeability layers, and improving the coordinated development of medium- and low-permeability layers. The authors posit that this strategy provides experimental evidence and engineering guidance for the late-stage development of comparable offshore heavy oil reservoirs.

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