Tight oil reservoirs have poor physical properties, and the problems including rapid oil rate decline and low oil recovery degree are quite common after volume fracturing. To obtain a general understanding of tight oil reservoir production improvement by CO₂ huff ‘n’ puff, the high-pressure physical properties of typical tight oil samples are measured. Combining the typical reservoir parameters, the production characteristics of the tight oil reservoir developed by the CO₂ huff ‘n’ puff are numerically studied on the basis of highly fitted experimental results. The results show that: (1) during the natural depletion stage, the oil production rate decreases rapidly and the oil recovery degree is low because of the decrease in oil displacement energy and the increase in fluid seepage resistance. (2) CO₂ huff ‘n’ puff can improve the development effect of tight oil reservoirs by supplementing reservoir energy and improving oil mobility, but the development effect gradually worsens with increasing cycle number. (3) The earlier the CO₂ injection timing is, the better the development effect of the tight reservoir is, but the less sufficient natural energy utilization is. When carrying out CO₂ stimulation, full use should be made of the natural energy, and the appropriate injection timing should be determined by comprehensively considering the formation-saturation pressure difference and oil production rate. The research results are helpful for strengthening the understanding of the production characteristics of tight oil reservoirs developed by CO₂ huff ‘n’ puff.

Focusing on problems of rapid production decline and poor development effect after volume fracturing of the shale oil reservoir, a production method of SC-HNP (surfactant slug assisting CO2 huff and puff) was proposed. Taking CQ block (CQ, the reservoir block name) as an example, laboratory experiments were carried out using typical oil samples and core samples, and a numerical simulation model of SC-HNP was established based on the experimental results. The production performance and oil recovery mechanism were analyzed, and the oil production by SC-HNP of a typical well was predicted. The results showed that (1) the oil recovery degree of the surfactant imbibition experiment is 34.5%, showing good imbibition oil recovery effect. CO2 has good swelling effect and viscosity reduction ability to the oil. (2) SC-HNP combines the effects of the surfactant and CO2, and it has the best replenishing formation energy ability and the highest oil production level. Its improved recovery degree is about 2.4% (one cycle), showing the best development effect. (3) Compared with SF-HNP (surfactant huff and puff), SC-HNP can improve the oil displacement effect in the far-well fractured area, and compared with CO2-HNP (CO2 huff and puff), SC-HNP can improve the oil displacement effect in the near-well fractured area. (4) Using SC-HNP, it is predicted that the oil recovery degree of well CQP-32 (CQP-32, the well name) can be increased by 1.84%, which significantly improves the development effect, and the CO2 storage percent is about 32%.

The depressurization effect is limited to class III hydrate reservoir recovery. To improve the depressurization effect, a new method of volume-fracturing and cyclic N₂ stimulation combination (VFCS) was proposed. The production performance of this method was investigated by using numerical models based on reservoir parameters of the SH7 hydrate site in the South China Sea. The results show that (1) VFCS can greatly enhance the production performance with the average CH₄ production rate being approximately 2.85 times higher than that of pure depressurization. This method combines the effects of volume fracturing and cyclic N₂ stimulation by improving the seepage environment and further reducing the CH₄ partial pressure in the gas phase. (2) High reservoir permeability, medium hydrate saturation, large volume-fracturing scale, low bottom-hole pressure, and high N₂ injection amount can increase CH₄ production by VFCS. (3) Although VFCS has the largest CH₄ production volume and the highest hydrate dissociation degree among the studied production strategies, the reservoir temperature drop is significant by VFCS and future studies can be focused on the external heat supply to the reservoir to further improve the production.

The cyclic methane hydrate production method was proposed with CO₂ and N₂ mixture stimulation. The cyclic production model was established based on actual hydrate reservoir parameters, accordingly, the production characteristics were analyzed, and a sensitivity analysis was conducted. The results show the following: (1) The depressurization mechanism is dominant in the cyclic production. CH₄ production and CH₄ hydrate dissociation can be greatly enhanced because the cyclic process can effectively reduce the partial pressure of CH₄ (gas phase). However, there is a limited effect for CO₂ storage. (2) Heat supply is essential for continuous hydrate dissociation. The CH₄ hydrate dissociation degree is the highest in the near-wellbore area; in addition, the fluid porosity and effective permeability are significantly improved, and the reservoir temperature is obviously decreased. (3) The initial CH₄ hydrate saturation, absolute permeability, intrinsic CO₂ hydrate formation kinetic constant, injection time and production time can significantly influence the production performance of the natural gas hydrate reservoir.

After volume fracturing of shale oil reservoirs, rapid oil rate decline and poor development performance are observed. Associated gas huff-and-puff has emerged as a promising method to enhance shale oil recovery. However, the influence patterns of key parameters on huff-and-puff performance remain poorly understood and lack systematic quantitative analysis. To address this, three categories (reservoir parameters, fracturing parameters, and operational parameters) with a total of 11 parameters were selected. Radial basis function neural network models were constructed under different parameter configurations, and global sensitivity analyses were performed using Sobol and extended Fourier Amplitude Sensitivity Test (EFAST) methods to quantify parameter influences on cumulative oil production and huff-and-puff oil production. Orthogonal experimental design was also applied to production prediction in a field case. Results indicate: (1) Sobol and EFAST methods provide consistent sensitivity analysis results. For cumulative oil production, oil saturation, effective thickness, fracturing scale, and permeability have the most significant direct effects, with strong interactions. For huff-and-puff oil production, oil saturation, injection timing, and effective thickness dominate with strong parameter interactions. (2) Under certain reservoir and fracturing conditions, gas injection volume and injection timing are the most critical parameters affecting huff-and-puff oil production, with notable interactions among the four operational parameters. (3) Favorable associated gas huff-and-puff production can be achieved with high injection volumes and rates, a relatively long soaking time, and moderate gas injection timing, resulting in an increased oil production by 31.2%. These findings can serve as a reference for optimizing strategies for developing shale oil reservoirs.