Produced Water Reinjection in China’s Oil and Gas Industry: Challenges, Standards, and Groundwater Protection
Oil and natural gas form the backbone of China’s energy system and are of critical importance to national energy security and overall socio-economic development.
According to data from the National Bureau of Statistics of China, in 2021 China’s domestic crude oil production reached nearly 199 million tonnes, while imports rose to 513 million tonnes. During the same period, natural gas production reached 205.3 billion m³, with imports of 168.7 billion m³. Under the evolving energy landscape, the strategy of “stabilizing oil production and increasing gas production” is regarded as a key pathway to ensuring and strengthening China’s oil and gas security. In the context of global climate change, natural gas—an important pillar for achieving China’s “dual-carbon” goals and the vision of a “Beautiful China”—is expected to play a central role in the green transition of the energy mix. By 2025, natural gas production is projected to exceed 230 billion m³, and after 2040 it is expected to stabilize above 300 billion m³, with oil and gas accounting for approximately 32% of total energy consumption. Given China’s unique resource endowment, the development of unconventional oil and gas resources will be the primary driving force behind this strategy. Statistics show that in 2020, unconventional natural gas accounted for 38% of total gas production, and this proportion is expected to exceed 50% by 2035.
With the intensification of unconventional oil and gas development and the entry of many fields into mid-to-late development stages, the issue of produced water has become increasingly prominent, with volumes rising sharply. Studies indicate that in late-stage oil and gas field development, water content in produced fluids typically reaches 70–80%, and in some cases exceeds 90%. During the early production stage of shale gas wells, daily produced water volumes can reach 20–30 m³ per well. Produced water is characterized by high salinity (30–360 g/L), elevated oil and suspended solids content, diverse organic and heavy-metal pollutants, and potential naturally occurring radioactive materials (NORM), making treatment both technically challenging and costly. Consequently, the industry commonly treats produced water through coagulation–sedimentation and filtration, followed by safe reinjection into active or depleted reservoirs, to maintain formation pressure, enhance recovery, and reduce discharge costs. To protect groundwater, reinjection wells are typically drilled to depths exceeding 1,000 m, with the deepest recorded well reaching 5,757 m. However, complex geological conditions increase the risk of leakage, which may contaminate groundwater or surface water through well integrity failure, fault migration, or caprock damage.
Given the complexity, concealment, lag effects, and difficulty of remediation associated with groundwater contamination, China has systematically promoted groundwater pollution prevention since 2011 through policies and regulations such as the National Groundwater Pollution Prevention and Control Plan and the Water Pollution Prevention and Control Action Plan. For produced water reinjection, the Ministry of Ecology and Environment and other authorities have issued multiple regulations emphasizing environmental feasibility assessments, pollution prevention, and monitoring requirements, while strictly prohibiting the reinjection of unrelated wastewater. In particular, the implementation of the Groundwater Management Regulation marks a new phase of legally enforced groundwater governance, imposing stricter management and pollution control requirements for produced water reinjection. As environmental thresholds continue to rise, further research on management systems, risk control standards, and technical specifications is urgently needed to achieve coordinated development between oil and gas production and groundwater protection. Notably, although several industry standards are in place, national-level or MEE-specific standards for produced water reinjection remain incomplete.
Reinjection site selection involves a detailed evaluation of both reinjection formations and wells. While produced water reinjection in oilfields aims primarily to enhance recovery, current practices and standards in China focus mainly on gas field produced water. Existing standards specify detailed criteria for formation and well selection. Key considerations for reinjection formations include:
a. Storage capacity, ensuring sufficient volume to accommodate the planned injection;
b. Sealing and isolation performance, requiring effective caprock and prevention of migration through faults or surface outcrops;
c. Reinjection depth, to avoid potential impacts on shallow drinking-water resources.
Reinjection well selection emphasizes wellbore integrity, coordination with field development and injection zones, and strict avoidance of groundwater-sensitive areas. As well integrity failure is a major risk factor for produced water leakage and shallow groundwater contamination, existing standards impose stringent requirements on well design, corrosion protection, and cementing quality.
Prior to reinjection, produced water must undergo strict water quality control to ensure compatibility with formation water and reservoir rock, preventing damage caused by water sensitivity (salinity, acid, or alkali sensitivity). Incompatible water quality may result in scaling precipitation, clay mineral swelling, and pore blockage, thereby altering reservoir structure, reducing injectivity, increasing operational difficulty, and in extreme cases leading to reinjection well failure or caprock integrity loss. Poor water quality can also accelerate corrosion and scaling in injection systems and pipelines, reducing equipment performance and service life.
Current domestic standards for gas field produced water reinjection focus on key indicators such as pH, dissolved oxygen, oil content, suspended solids, sulfate-reducing bacteria, and iron bacteria, with detailed specifications provided in CNPC enterprise standards. In contrast, industry standards issued in the same period emphasize compatibility and injectivity, without specifying exact parameter limits. For oilfield reinjection, where pressure maintenance and enhanced recovery are critical, water quality requirements are generally more stringent.
To mitigate potential risks to groundwater, existing standards also strengthen operational management and shallow water monitoring. Operational management includes recording injection parameters (pressure, volume, and water quality) and regular inspection of well and injection system conditions. Monitoring focuses on dynamic surveillance of water quality in the reinjection area to ensure early detection and timely response to anomalies, thereby ensuring environmentally safe reinjection operations.
